From d2d9f6ced6d087b22d8c798462543d4ce8fdd5ad Mon Sep 17 00:00:00 2001
From: scruel <scruelt@hotmail.com>
Date: Thu, 11 Jan 2018 13:39:52 +0800
Subject: [PATCH] finish 6.4 & 6.5

---
 image/20180111_080314.png | Bin 0 -> 9669 bytes
 image/20180111_080514.png | Bin 0 -> 8774 bytes
 image/20180111_080614.png | Bin 0 -> 52430 bytes
 week1.html                |   2 +-
 week1.md                  |  14 +++---
 week2.html                |   4 +-
 week2.md                  |  24 +++++----
 week3.html                |  11 ++++-
 week3.md                  | 123 +++++++++++++++++++++++++++++++++++++++-------
 9 files changed, 138 insertions(+), 40 deletions(-)
 create mode 100644 image/20180111_080314.png
 create mode 100644 image/20180111_080514.png
 create mode 100644 image/20180111_080614.png

diff --git a/image/20180111_080314.png b/image/20180111_080314.png
new file mode 100644
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 <div  id='write'  class = 'is-node show-fences-line-number'><div class='md-toc' mdtype='toc'><p class="md-toc-content"><span class="md-toc-item md-toc-h1" data-ref="n2"><a class="md-toc-inner" href="#header-n2">1. 引言(Introduction)</a></span><span class="md-toc-item md-toc-h2" data-ref="n3"><a class="md-toc-inner" href="#header-n3">1.1 Welcome</a></span><span class="md-toc-item md-toc-h2" data-ref="n57"><a class="md-toc-inner" href="#header-n57">1.2 什么是机器学习(What is Machine Learning)</a></span><span class="md-toc-item md-toc-h2" data-ref="n113"><a class="md-toc-inner" href="#header-n113">1.3 监督学习(Supervised Learning)</a></span><span class="md-toc-item md-toc-h2" data-ref="n142"><a class="md-toc-inner" href="#header-n142">1.4 无监督学习(Unsupervised Learning)</a></span><span class="md-toc-item md-toc-h1" data-ref="n199"><a class="md-toc-inner" href="#header-n199">2 单变量线性回归(Linear Regression with One Variable)</a></span><span class="md-toc-item md-toc-h2" data-ref="n200"><a class="md-toc-inner" href="#header-n200">2.1 模型表示(Model Representation)</a></span><span class="md-toc-item md-toc-h2" data-ref="n243"><a class="md-toc-inner" href="#header-n243">2.2 损失函数(Cost Function)</a></span><span class="md-toc-item md-toc-h2" data-ref="n270"><a class="md-toc-inner" href="#header-n270">2.3 损失函数 - 直观理解1(Cost Function - Intuition I)</a></span><span class="md-toc-item md-toc-h2" data-ref="n292"><a class="md-toc-inner" href="#header-n292">2.4 损失函数 - 直观理解2(Cost Function - Intuition II)</a></span><span class="md-toc-item md-toc-h2" data-ref="n316"><a class="md-toc-inner" href="#header-n316">2.5 梯度下降(Gradient Descent)</a></span><span class="md-toc-item md-toc-h2" data-ref="n346"><a class="md-toc-inner" href="#header-n346">2.6 梯度下降直观理解(Gradient Descent Intuition)</a></span><span class="md-toc-item md-toc-h2" data-ref="n385"><a class="md-toc-inner" href="#header-n385">2.7 线性回归中的梯度下降(Gradient Descent For Linear Regression)</a></span><span class="md-toc-item md-toc-h1" data-ref="n431"><a class="md-toc-inner" href="#header-n431">3 Linear Algebra Review</a></span><span class="md-toc-item md-toc-h2" data-ref="n434"><a class="md-toc-inner" href="#header-n434">3.1 Matrices and Vectors</a></span><span class="md-toc-item md-toc-h2" data-ref="n441"><a class="md-toc-inner" href="#header-n441">3.2 Addition and Scalar Multiplication</a></span><span class="md-toc-item md-toc-h2" data-ref="n448"><a class="md-toc-inner" href="#header-n448">3.3 Matrix Vector Multiplication</a></span><span class="md-toc-item md-toc-h2" data-ref="n455"><a class="md-toc-inner" href="#header-n455">3.4 Matrix Matrix Multiplication</a></span><span class="md-toc-item md-toc-h2" data-ref="n462"><a class="md-toc-inner" href="#header-n462">3.5 Matrix Multiplication Properties</a></span><span class="md-toc-item md-toc-h2" data-ref="n469"><a class="md-toc-inner" href="#header-n469">3.6 Inverse and Transpose</a></span></p></div><h1><a name='header-n2' class='md-header-anchor '></a>1. 引言(Introduction)</h1><h2><a name='header-n3' class='md-header-anchor '></a>1.1 Welcome</h2><p>随着互联网数据不断累积，硬件不断升级迭代，在这个信息爆炸的时代，机器学习已被应用在各行各业中，可谓无处不在。</p><p>一些常见的机器学习的应用，例如：</p><ul><li>手写识别</li><li>垃圾邮件分类</li><li>搜索引擎</li><li>图像处理</li><li>…</li></ul><p>使用到机器学习的一些案例：</p><ul><li><p>数据挖掘</p><ul><li>网页点击流数据分析</li></ul></li><li><p>人工无法处理的工作(量大)</p><ul><li>手写识别</li><li>计算机视觉</li></ul></li><li><p>个人定制</p><ul><li>推荐系统</li></ul></li><li><p>研究大脑</p></li><li><p>……</p></li></ul><h2><a name='header-n57' class='md-header-anchor '></a>1.2 什么是机器学习(What is Machine Learning)</h2><ol start='' ><li><p>机器学习定义
 这里主要有两种定义：</p><ul><li><p>Arthur Samuel (1959). Machine Learning: Field of study that gives computers the ability to learn without being explicitly programmed.</p><p>这个定义有点不正式但提出的时间最早，来自于一个懂得计算机编程的下棋菜鸟。他编写了一个程序，但没有显式地编程每一步该怎么走，而是让计算机自己和自己对弈，并不断地计算布局的好坏，来判断什么情况下获胜的概率高，从而积累经验，好似学习，最后，这个计算机程序成为了一个比他自己还厉害的棋手。</p></li><li><p>Tom Mitchell (1998) Well-posed Learning Problem: A computer program is said to learn from experience E with respect to some <strong>task T</strong> and some <strong>performance measure P</strong>, if its performance on T, as measured by P, improves with <strong>experience E</strong>. </p><p>Tom Mitchell 的定义更为现代和正式。在过滤垃圾邮件这个例子中，电子邮件系统会根据用户对电子邮件的标记（是/不是垃圾邮件）不断学习，从而提升过滤垃圾邮件的准确率，定义中的三个字母分别代表：</p><ul><li>T(Task): 过滤垃圾邮件任务。</li><li>P(Performance): 电子邮件系统过滤垃圾邮件的准确率。</li><li>E(Experience): 用户对电子邮件的标记。</li></ul></li></ul></li><li><p>机器学习算法</p><p>主要有两种机器学习的算法分类</p><ol start='' ><li>监督学习</li><li>无监督学习</li></ol><p>两者的区别为<strong>是否需要人工参与数据结果的标注</strong>。这两部分的内容占比很大，并且很重要，掌握好了可以在以后的应用中节省大把大把的时间~</p><p>还有一些算法也属于机器学习领域，诸如：</p><ul><li>半监督学习: 介于监督学习于无监督学习之间</li><li>推荐算法: 没错，就是那些个买完某商品后还推荐同款的某购物网站所用的算法。</li><li>强化学习: 通过观察来学习如何做出动作，每个动作都会对环境有所影响，而环境的反馈又可以引导该学习算法。</li><li>迁移学习</li></ul></li></ol><h2><a name='header-n113' class='md-header-anchor '></a>1.3 监督学习(Supervised Learning)</h2><p>监督学习，即为教计算机如何去完成预测任务（有反馈），预先给一定数据量的输入<strong>和对应的结果</strong>即训练集，建模拟合，最后让计算机预测未知数据的结果。</p><p>监督学习一般有两种：</p><ol start='' ><li><p>回归问题(Regression)</p><p>回归问题即为预测一系列的<strong>连续值</strong>。</p><p>在房屋价格预测的例子中，给出了一系列的房屋面基数据，根据这些数据来预测任意面积的房屋价格。给出照片-年龄数据集，预测给定照片的年龄。</p><p><img src='image/20180105_194712.png' alt='' /></p></li><li><p>分类问题(Classification)</p><p>分类问题即为预测一系列的<strong>离散值</strong>。</p><p>即根据数据预测被预测对象属于哪个分类。</p><p>视频中举了癌症肿瘤这个例子，针对诊断结果，分别分类为良性或恶性。还例如垃圾邮件分类问题，也同样属于监督学习中的分类问题。</p><p><img src='image/20180105_194839.png' alt='' /></p></li></ol><p>视频中提到<strong>支持向量机</strong>这个算法，旨在解决当特征量很大的时候(特征即如癌症例子中的肿块大小，颜色，气味等各种特征)，计算机内存一定会不够用的情况。<strong>支持向量机能让计算机处理无限多个特征。</strong></p><h2><a name='header-n142' class='md-header-anchor '></a>1.4 无监督学习(Unsupervised Learning)</h2><p>相对于监督学习，训练集不会有人为标注的结果（无反馈），我们<strong>不会给出</strong>结果或<strong>无法得知</strong>训练集的结果是什么样，而是单纯由计算机通过无监督学习算法自行分析，从而“得出结果”。计算机可能会把特定的数据集归为几个不同的簇，故叫做聚类算法。</p><p>无监督学习一般分为两种：</p><ol start='' ><li><p>聚类(Clustering)</p><ul><li>新闻聚合</li><li>DNA 个体聚类</li><li>天文数据分析</li><li>市场细分</li><li>社交网络分析</li></ul></li><li><p>非聚类(Non-clustering)</p><ul><li>鸡尾酒问题</li></ul></li></ol><p><strong>新闻聚合</strong></p><p>在例如谷歌新闻这样的网站中，每天后台都会收集成千上万的新闻，然后将这些新闻分组成一个个的新闻专题，这样一个又一个聚类，就是应用了无监督学习的结果。</p><p><strong>鸡尾酒问题</strong></p><p><img src='image/20180105_201639.png' alt='' /></p><p>在鸡尾酒会上，大家说话声音彼此重叠，几乎很难分辨出面前的人说了什么。我们很难对于这个问题进行数据标注，而这里的通过机器学习的无监督学习算法，就可以将说话者的声音同背景音乐分离出来，看视频，效果还不错呢~~。</p><p>嗯，这块是打打鸡血的，只需要一行代码就解决了问题，就是这么简单！当然，我没复现过 ^_^……</p><p>神奇的一行代码：
 <code>[W,s,v] = svd((repmat(sum(x.*x,1),size(x,1),1).*x)*x&#39;);</code></p><p><strong>编程语言建议</strong></p><p>在机器学习刚开始时，<strong>推荐使用 Octave 类的工程计算编程软件</strong>，因为在 C++ 或 Java 等编程语言中，编写对应的代码需要用到复杂的库以及要写大量的冗余代码，比较耗费时间，建议可以在学习过后再考虑使用其他语言来构建系统。<strong></strong>
-另外，在做<strong>原型搭建</strong>的时候也应该先考虑使用类似于 Octave 这种便于计算的编程软件，当其已经可以工作后，才将模型移植到其他的高级编程语言中。</p><blockquote><p>注：Octave 与 MATLAB 语法相近，由于 MATLAB 为商业软件，课程中使用开源且免费的 Octave。</p><p>机器学习领域发展迅速，现在也可使用 Tensorflow 等开源机器学习框架编写机器学习代码，这些框架十分友好，易于编写及应用。</p></blockquote><h1><a name='header-n199' class='md-header-anchor '></a>2 单变量线性回归(Linear Regression with One Variable)</h1><h2><a name='header-n200' class='md-header-anchor '></a>2.1 模型表示(Model Representation)</h2><ol start='' ><li><p>房价预测训练集</p><figure><table><thead><tr><th>Size in <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-1-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="5.339ex" height="2.757ex" viewBox="0 -889.4 2298.9 1187.2" role="img" focusable="false" style="vertical-align: -0.692ex;"><defs><path stroke-width="1" id="E1-MJMATHI-66" d="M118 -162Q120 -162 124 -164T135 -167T147 -168Q160 -168 171 -155T187 -126Q197 -99 221 27T267 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class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-42-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="1.339ex" height="1.994ex" viewBox="0 -755.9 576.5 858.4" role="img" focusable="false" style="vertical-align: -0.238ex;"><defs><path stroke-width="1" id="E42-MJMATHI-68" d="M137 683Q138 683 209 688T282 694Q294 694 294 685Q294 674 258 534Q220 386 220 383Q220 381 227 388Q288 442 357 442Q411 442 444 415T478 336Q478 285 440 178T402 50Q403 36 407 31T422 26Q450 26 474 56T513 138Q516 149 519 151T535 153Q555 153 555 145Q555 144 551 130Q535 71 500 33Q466 -10 419 -10H414Q367 -10 346 17T325 74Q325 90 361 192T398 345Q398 404 354 404H349Q266 404 205 306L198 293L164 158Q132 28 127 16Q114 -11 83 -11Q69 -11 59 -2T48 16Q48 30 121 320L195 616Q195 629 188 632T149 637H128Q122 643 122 645T124 664Q129 683 137 683Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" 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419 -10H414Q367 -10 346 17T325 74Q325 90 361 192T398 345Q398 404 354 404H349Q266 404 205 306L198 293L164 158Q132 28 127 16Q114 -11 83 -11Q69 -11 59 -2T48 16Q48 30 121 320L195 616Q195 629 188 632T149 637H128Q122 643 122 645T124 664Q129 683 137 683Z"></path><path stroke-width="1" id="E72-MJMATHI-3B8" d="M35 200Q35 302 74 415T180 610T319 704Q320 704 327 704T339 705Q393 701 423 656Q462 596 462 495Q462 380 417 261T302 66T168 -10H161Q125 -10 99 10T60 63T41 130T35 200ZM383 566Q383 668 330 668Q294 668 260 623T204 521T170 421T157 371Q206 370 254 370L351 371Q352 372 359 404T375 484T383 566ZM113 132Q113 26 166 26Q181 26 198 36T239 74T287 161T335 307L340 324H145Q145 321 136 286T120 208T113 132Z"></path><path stroke-width="1" id="E72-MJMAIN-28" d="M94 250Q94 319 104 381T127 488T164 576T202 643T244 695T277 729T302 750H315H319Q333 750 333 741Q333 738 316 720T275 667T226 581T184 443T167 250T184 58T225 -81T274 -167T316 -220T333 -241Q333 -250 318 -250H315H302L274 -226Q180 -141 137 -14T94 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type="math/tex" id="MathJax-Element-13">x</script>: 特征/输入变量。</p></blockquote><p>上式中，<span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-69-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="1.09ex" height="2.11ex" viewBox="0 -806.1 469.5 908.7" role="img" focusable="false" style="vertical-align: -0.238ex;"><defs><path stroke-width="1" id="E69-MJMATHI-3B8" d="M35 200Q35 302 74 415T180 610T319 704Q320 704 327 704T339 705Q393 701 423 656Q462 596 462 495Q462 380 417 261T302 66T168 -10H161Q125 -10 99 10T60 63T41 130T35 200ZM383 566Q383 668 330 668Q294 668 260 623T204 521T170 421T157 371Q206 370 254 370L351 371Q352 372 359 404T375 484T383 566ZM113 132Q113 26 166 26Q181 26 198 36T239 74T287 161T335 307L340 324H145Q145 321 136 286T120 208T113 132Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E69-MJMATHI-3B8" x="0" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-69">\theta</script> 为参数，<span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-69-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="1.09ex" height="2.11ex" viewBox="0 -806.1 469.5 908.7" role="img" focusable="false" style="vertical-align: -0.238ex;"><defs><path stroke-width="1" id="E69-MJMATHI-3B8" d="M35 200Q35 302 74 415T180 610T319 704Q320 704 327 704T339 705Q393 701 423 656Q462 596 462 495Q462 380 417 261T302 66T168 -10H161Q125 -10 99 10T60 63T41 130T35 200ZM383 566Q383 668 330 668Q294 668 260 623T204 521T170 421T157 371Q206 370 254 370L351 371Q352 372 359 404T375 484T383 566ZM113 132Q113 26 166 26Q181 26 198 36T239 74T287 161T335 307L340 324H145Q145 321 136 286T120 208T113 132Z"></path></defs><g stroke="currentColor" 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id="E69-MJMATHI-3B8" d="M35 200Q35 302 74 415T180 610T319 704Q320 704 327 704T339 705Q393 701 423 656Q462 596 462 495Q462 380 417 261T302 66T168 -10H161Q125 -10 99 10T60 63T41 130T35 200ZM383 566Q383 668 330 668Q294 668 260 623T204 521T170 421T157 371Q206 370 254 370L351 371Q352 372 359 404T375 484T383 566ZM113 132Q113 26 166 26Q181 26 198 36T239 74T287 161T335 307L340 324H145Q145 321 136 286T120 208T113 132Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E69-MJMATHI-3B8" x="0" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-69">\theta</script> 以更好地拟合数据，成了求解该问题的最终问题。</p><p>单变量，即只有一个特征(如例子中房屋的面积这个特征)。</p><h2><a name='header-n243' class='md-header-anchor '></a>2.2 损失函数(Cost Function)</h2><p>我们的目的在于求解预测结果 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-15-Frame" tabindex="-1" style="font-size: 100%; display: 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250 629Q208 629 178 597Q153 571 145 525T137 333Q137 175 145 125T181 46Q209 16 250 16Q290 16 318 46Q347 76 354 130T362 333Q362 478 354 524T321 597Z"></path><path stroke-width="1" id="E29-MJMAIN-2C" d="M78 35T78 60T94 103T137 121Q165 121 187 96T210 8Q210 -27 201 -60T180 -117T154 -158T130 -185T117 -194Q113 -194 104 -185T95 -172Q95 -168 106 -156T131 -126T157 -76T173 -3V9L172 8Q170 7 167 6T161 3T152 1T140 0Q113 0 96 17Z"></path><path stroke-width="1" id="E29-MJMAIN-31" d="M213 578L200 573Q186 568 160 563T102 556H83V602H102Q149 604 189 617T245 641T273 663Q275 666 285 666Q294 666 302 660V361L303 61Q310 54 315 52T339 48T401 46H427V0H416Q395 3 257 3Q121 3 100 0H88V46H114Q136 46 152 46T177 47T193 50T201 52T207 57T213 61V578Z"></path><path stroke-width="1" id="E29-MJMAIN-29" d="M60 749L64 750Q69 750 74 750H86L114 726Q208 641 251 514T294 250Q294 182 284 119T261 12T224 -76T186 -143T145 -194T113 -227T90 -246Q87 -249 86 -250H74Q66 -250 63 -250T58 -247T55 -238Q56 -237 66 -225Q221 -64 221 250T66 725Q56 737 55 738Q55 746 60 749Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E29-MJMATHI-4A" x="0" y="0"></use><g transform="translate(800,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E29-MJMAIN-28" x="0" y="0"></use><g transform="translate(389,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E29-MJMATHI-3B8" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E29-MJMAIN-30" x="663" y="-213"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E29-MJMAIN-2C" x="1312" y="0"></use><g transform="translate(1758,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E29-MJMATHI-3B8" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E29-MJMAIN-31" x="663" y="-213"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E29-MJMAIN-29" x="2681" y="0"></use></g></g></svg></span><script type="math/tex" id="MathJax-Element-29">J\left( \theta_0, \theta_1  \right)</script> 的最小值</strong>。</p><p>&nbsp;</p><h2><a name='header-n270' class='md-header-anchor '></a>2.3 损失函数 - 直观理解1(Cost Function - Intuition I)</h2><p>根据上节视频，列出如下定义：</p><ul><li>假设函数(Hypothesis): <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-72-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="17.039ex" height="2.577ex" viewBox="0 -806.1 7336.3 1109.7" role="img" focusable="false" style="vertical-align: -0.705ex;"><defs><path stroke-width="1" id="E72-MJMATHI-68" d="M137 683Q138 683 209 688T282 694Q294 694 294 685Q294 674 258 534Q220 386 220 383Q220 381 227 388Q288 442 357 442Q411 442 444 415T478 336Q478 285 440 178T402 50Q403 36 407 31T422 26Q450 26 474 56T513 138Q516 149 519 151T535 153Q555 153 555 145Q555 144 551 130Q535 71 500 33Q466 -10 419 -10H414Q367 -10 346 17T325 74Q325 90 361 192T398 345Q398 404 354 404H349Q266 404 205 306L198 293L164 158Q132 28 127 16Q114 -11 83 -11Q69 -11 59 -2T48 16Q48 30 121 320L195 616Q195 629 188 632T149 637H128Q122 643 122 645T124 664Q129 683 137 683Z"></path><path stroke-width="1" id="E72-MJMATHI-3B8" d="M35 200Q35 302 74 415T180 610T319 704Q320 704 327 704T339 705Q393 701 423 656Q462 596 462 495Q462 380 417 261T302 66T168 -10H161Q125 -10 99 10T60 63T41 130T35 200ZM383 566Q383 668 330 668Q294 668 260 623T204 521T170 421T157 371Q206 370 254 370L351 371Q352 372 359 404T375 484T383 566ZM113 132Q113 26 166 26Q181 26 198 36T239 74T287 161T335 307L340 324H145Q145 321 136 286T120 208T113 132Z"></path><path stroke-width="1" id="E72-MJMAIN-28" d="M94 250Q94 319 104 381T127 488T164 576T202 643T244 695T277 729T302 750H315H319Q333 750 333 741Q333 738 316 720T275 667T226 581T184 443T167 250T184 58T225 -81T274 -167T316 -220T333 -241Q333 -250 318 -250H315H302L274 -226Q180 -141 137 -14T94 250Z"></path><path stroke-width="1" id="E72-MJMATHI-78" d="M52 289Q59 331 106 386T222 442Q257 442 286 424T329 379Q371 442 430 442Q467 442 494 420T522 361Q522 332 508 314T481 292T458 288Q439 288 427 299T415 328Q415 374 465 391Q454 404 425 404Q412 404 406 402Q368 386 350 336Q290 115 290 78Q290 50 306 38T341 26Q378 26 414 59T463 140Q466 150 469 151T485 153H489Q504 153 504 145Q504 144 502 134Q486 77 440 33T333 -11Q263 -11 227 52Q186 -10 133 -10H127Q78 -10 57 16T35 71Q35 103 54 123T99 143Q142 143 142 101Q142 81 130 66T107 46T94 41L91 40Q91 39 97 36T113 29T132 26Q168 26 194 71Q203 87 217 139T245 247T261 313Q266 340 266 352Q266 380 251 392T217 404Q177 404 142 372T93 290Q91 281 88 280T72 278H58Q52 284 52 289Z"></path><path stroke-width="1" id="E72-MJMAIN-29" d="M60 749L64 750Q69 750 74 750H86L114 726Q208 641 251 514T294 250Q294 182 284 119T261 12T224 -76T186 -143T145 -194T113 -227T90 -246Q87 -249 86 -250H74Q66 -250 63 -250T58 -247T55 -238Q56 -237 66 -225Q221 -64 221 250T66 725Q56 737 55 738Q55 746 60 749Z"></path><path stroke-width="1" id="E72-MJMAIN-3D" d="M56 347Q56 360 70 367H707Q722 359 722 347Q722 336 708 328L390 327H72Q56 332 56 347ZM56 153Q56 168 72 173H708Q722 163 722 153Q722 140 707 133H70Q56 140 56 153Z"></path><path stroke-width="1" id="E72-MJMAIN-30" d="M96 585Q152 666 249 666Q297 666 345 640T423 548Q460 465 460 320Q460 165 417 83Q397 41 362 16T301 -15T250 -22Q224 -22 198 -16T137 16T82 83Q39 165 39 320Q39 494 96 585ZM321 597Q291 629 250 629Q208 629 178 597Q153 571 145 525T137 333Q137 175 145 125T181 46Q209 16 250 16Q290 16 318 46Q347 76 354 130T362 333Q362 478 354 524T321 597Z"></path><path stroke-width="1" id="E72-MJMAIN-2B" d="M56 237T56 250T70 270H369V420L370 570Q380 583 389 583Q402 583 409 568V270H707Q722 262 722 250T707 230H409V-68Q401 -82 391 -82H389H387Q375 -82 369 -68V230H70Q56 237 56 250Z"></path><path stroke-width="1" id="E72-MJMAIN-31" d="M213 578L200 573Q186 568 160 563T102 556H83V602H102Q149 604 189 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xlink:href="#E72-MJMATHI-3B8" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E72-MJMAIN-30" x="663" y="-213"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E72-MJMAIN-2B" x="4839" y="0"></use><g transform="translate(5840,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E72-MJMATHI-3B8" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E72-MJMAIN-31" x="663" y="-213"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E72-MJMATHI-78" x="6763" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-72">h_\theta(x)=\theta_0+\theta_1x</script></li><li>参数(Parameters): <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-46-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="5.323ex" height="2.461ex" viewBox="0 -806.1 2292 1059.4" role="img" focusable="false" style="vertical-align: -0.588ex;"><defs><path stroke-width="1" id="E46-MJMATHI-3B8" d="M35 200Q35 302 74 415T180 610T319 704Q320 704 327 704T339 705Q393 701 423 656Q462 596 462 495Q462 380 417 261T302 66T168 -10H161Q125 -10 99 10T60 63T41 130T35 200ZM383 566Q383 668 330 668Q294 668 260 623T204 521T170 421T157 371Q206 370 254 370L351 371Q352 372 359 404T375 484T383 566ZM113 132Q113 26 166 26Q181 26 198 36T239 74T287 161T335 307L340 324H145Q145 321 136 286T120 208T113 132Z"></path><path stroke-width="1" id="E46-MJMAIN-30" d="M96 585Q152 666 249 666Q297 666 345 640T423 548Q460 465 460 320Q460 165 417 83Q397 41 362 16T301 -15T250 -22Q224 -22 198 -16T137 16T82 83Q39 165 39 320Q39 494 96 585ZM321 597Q291 629 250 629Q208 629 178 597Q153 571 145 525T137 333Q137 175 145 125T181 46Q209 16 250 16Q290 16 318 46Q347 76 354 130T362 333Q362 478 354 524T321 597Z"></path><path stroke-width="1" id="E46-MJMAIN-2C" d="M78 35T78 60T94 103T137 121Q165 121 187 96T210 8Q210 -27 201 -60T180 -117T154 -158T130 -185T117 -194Q113 -194 104 -185T95 -172Q95 -168 106 -156T131 -126T157 -76T173 -3V9L172 8Q170 7 167 6T161 3T152 1T140 0Q113 0 96 17Z"></path><path stroke-width="1" id="E46-MJMAIN-31" d="M213 578L200 573Q186 568 160 563T102 556H83V602H102Q149 604 189 617T245 641T273 663Q275 666 285 666Q294 666 302 660V361L303 61Q310 54 315 52T339 48T401 46H427V0H416Q395 3 257 3Q121 3 100 0H88V46H114Q136 46 152 46T177 47T193 50T201 52T207 57T213 61V578Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E46-MJMATHI-3B8" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E46-MJMAIN-30" x="663" y="-213"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E46-MJMAIN-2C" x="923" y="0"></use><g transform="translate(1368,0)"><use 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id="MathJax-Element-73"> J\left( \theta_0, \theta_1  \right)=\frac{1}{2m}\sum\limits_{i=1}^{m}{{{\left( {{h}_{\theta }}\left( {{x}^{(i)}} \right)-{{y}^{(i)}} \right)}^{2}}} </script></li><li>目标(Goal): <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-33-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="18.161ex" height="4.212ex" viewBox="0 -806.1 7819.1 1813.3" role="img" focusable="false" style="vertical-align: -2.339ex;"><defs><path stroke-width="1" id="E33-MJMAIN-6D" d="M41 46H55Q94 46 102 60V68Q102 77 102 91T102 122T103 161T103 203Q103 234 103 269T102 328V351Q99 370 88 376T43 385H25V408Q25 431 27 431L37 432Q47 433 65 434T102 436Q119 437 138 438T167 441T178 442H181V402Q181 364 182 364T187 369T199 384T218 402T247 421T285 437Q305 442 336 442Q351 442 364 440T387 434T406 426T421 417T432 406T441 395T448 384T452 374T455 366L457 361L460 365Q463 369 466 373T475 384T488 397T503 410T523 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y="-213"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E33-MJMAIN-2C" x="1312" y="0"></use><g transform="translate(1758,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E33-MJMATHI-3B8" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E33-MJMAIN-31" x="663" y="-213"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E33-MJMAIN-29" x="2681" y="0"></use></g></g></svg></span><script type="math/tex" id="MathJax-Element-33">\underset{\theta_0, \theta_1}{\text{minimize}} J \left(\theta_0, \theta_1 \right)</script></li></ul><p>为了直观理解损失函数到底是在做什么，先假设 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-34-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="6.406ex" height="2.344ex" viewBox="0 -806.1 2758 1009.2" role="img" focusable="false" style="vertical-align: 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y="0"></use></g></g></svg></span><script type="math/tex" id="MathJax-Element-50">h_\theta\left(x\right)</script> ，并分析 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-38-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="8.991ex" height="2.577ex" viewBox="0 -806.1 3871.1 1109.7" role="img" focusable="false" style="vertical-align: -0.705ex;"><defs><path stroke-width="1" id="E38-MJMATHI-4A" d="M447 625Q447 637 354 637H329Q323 642 323 645T325 664Q329 677 335 683H352Q393 681 498 681Q541 681 568 681T605 682T619 682Q633 682 633 672Q633 670 630 658Q626 642 623 640T604 637Q552 637 545 623Q541 610 483 376Q420 128 419 127Q397 64 333 21T195 -22Q137 -22 97 8T57 88Q57 130 80 152T132 174Q177 174 182 130Q182 98 164 80T123 56Q115 54 115 53T122 44Q148 15 197 15Q235 15 271 47T324 130Q328 142 387 380T447 625Z"></path><path stroke-width="1" id="E38-MJMAIN-28" d="M94 250Q94 319 104 381T127 488T164 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y="0"></use></g></g></svg></span><script type="math/tex" id="MathJax-Element-38">J\left(\theta_0, \theta_1\right)</script> 随着 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-68-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="2.145ex" height="2.344ex" viewBox="0 -806.1 923.4 1009.2" role="img" focusable="false" style="vertical-align: -0.472ex;"><defs><path stroke-width="1" id="E68-MJMATHI-3B8" d="M35 200Q35 302 74 415T180 610T319 704Q320 704 327 704T339 705Q393 701 423 656Q462 596 462 495Q462 380 417 261T302 66T168 -10H161Q125 -10 99 10T60 63T41 130T35 200ZM383 566Q383 668 330 668Q294 668 260 623T204 521T170 421T157 371Q206 370 254 370L351 371Q352 372 359 404T375 484T383 566ZM113 132Q113 26 166 26Q181 26 198 36T239 74T287 161T335 307L340 324H145Q145 321 136 286T120 208T113 132Z"></path><path stroke-width="1" id="E68-MJMAIN-31" d="M213 578L200 573Q186 568 160 563T102 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xlink:href="#E41-MJMATHI-3B8" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E41-MJMAIN-30" x="663" y="-213"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E41-MJMAIN-2C" x="1312" y="0"></use><g transform="translate(1758,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E41-MJMATHI-3B8" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E41-MJMAIN-31" x="663" y="-213"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E41-MJMAIN-29" x="2681" y="0"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E41-MJMAIN-3D" x="4148" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E41-MJMAIN-30" x="5205" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-41">J\left(\theta_0, \theta_1 \right) = 0</script>，取得最小值，</strong>对应于左图青色直线，即函数 <span 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transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E42-MJMATHI-68" x="0" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-42">h</script> 拟合程度最好的情况。</p><h2><a name='header-n292' class='md-header-anchor '></a>2.4 损失函数 - 直观理解2(Cost Function - Intuition II)</h2><blockquote><p>注：该部分由于涉及到了多变量成像，可能较难理解，要求只需要理解上节内容即可，该节如果不能较好理解可跳过。</p></blockquote><p>给定数据集：</p><p><img src='image/20180106_091307.png' alt='' /></p><p>参数在 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-43-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="2.145ex" height="2.461ex" viewBox="0 -806.1 923.4 1059.4" role="img" focusable="false" style="vertical-align: -0.588ex;"><defs><path stroke-width="1" id="E43-MJMATHI-3B8" d="M35 200Q35 302 74 415T180 610T319 704Q320 704 327 704T339 705Q393 701 423 656Q462 596 462 495Q462 380 417 261T302 66T168 -10H161Q125 -10 99 10T60 63T41 130T35 200ZM383 566Q383 668 330 668Q294 668 260 623T204 521T170 421T157 371Q206 370 254 370L351 371Q352 372 359 404T375 484T383 566ZM113 132Q113 26 166 26Q181 26 198 36T239 74T287 161T335 307L340 324H145Q145 321 136 286T120 208T113 132Z"></path><path stroke-width="1" id="E43-MJMAIN-30" d="M96 585Q152 666 249 666Q297 666 345 640T423 548Q460 465 460 320Q460 165 417 83Q397 41 362 16T301 -15T250 -22Q224 -22 198 -16T137 16T82 83Q39 165 39 320Q39 494 96 585ZM321 597Q291 629 250 629Q208 629 178 597Q153 571 145 525T137 333Q137 175 145 125T181 46Q209 16 250 16Q290 16 318 46Q347 76 354 130T362 333Q362 478 354 524T321 597Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E43-MJMATHI-3B8" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E43-MJMAIN-30" x="663" y="-213"></use></g></svg></span><script type="math/tex" id="MathJax-Element-43">\theta_0</script> 不恒为 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-44-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="1.162ex" height="1.994ex" viewBox="0 -755.9 500.5 858.4" role="img" focusable="false" style="vertical-align: -0.238ex;"><defs><path stroke-width="1" id="E44-MJMAIN-30" d="M96 585Q152 666 249 666Q297 666 345 640T423 548Q460 465 460 320Q460 165 417 83Q397 41 362 16T301 -15T250 -22Q224 -22 198 -16T137 16T82 83Q39 165 39 320Q39 494 96 585ZM321 597Q291 629 250 629Q208 629 178 597Q153 571 145 525T137 333Q137 175 145 125T181 46Q209 16 250 16Q290 16 318 46Q347 76 354 130T362 333Q362 478 354 524T321 597Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E44-MJMAIN-30" x="0" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-44">0</script> 时损失函数 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-66-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="4.758ex" height="2.577ex" viewBox="0 -806.1 2048.7 1109.7" role="img" focusable="false" style="vertical-align: -0.705ex;"><defs><path stroke-width="1" id="E66-MJMATHI-4A" d="M447 625Q447 637 354 637H329Q323 642 323 645T325 664Q329 677 335 683H352Q393 681 498 681Q541 681 568 681T605 682T619 682Q633 682 633 672Q633 670 630 658Q626 642 623 640T604 637Q552 637 545 623Q541 610 483 376Q420 128 419 127Q397 64 333 21T195 -22Q137 -22 97 8T57 88Q57 130 80 152T132 174Q177 174 182 130Q182 98 164 80T123 56Q115 54 115 53T122 44Q148 15 197 15Q235 15 271 47T324 130Q328 142 387 380T447 625Z"></path><path stroke-width="1" id="E66-MJMAIN-28" d="M94 250Q94 319 104 381T127 488T164 576T202 643T244 695T277 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17Z"></path><path stroke-width="1" id="E46-MJMAIN-31" d="M213 578L200 573Q186 568 160 563T102 556H83V602H102Q149 604 189 617T245 641T273 663Q275 666 285 666Q294 666 302 660V361L303 61Q310 54 315 52T339 48T401 46H427V0H416Q395 3 257 3Q121 3 100 0H88V46H114Q136 46 152 46T177 47T193 50T201 52T207 57T213 61V578Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E46-MJMATHI-3B8" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E46-MJMAIN-30" x="663" y="-213"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E46-MJMAIN-2C" x="923" y="0"></use><g transform="translate(1368,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E46-MJMATHI-3B8" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E46-MJMAIN-31" x="663" y="-213"></use></g></g></svg></span><script type="math/tex" id="MathJax-Element-46">\theta_0, \theta_1</script> 的3-D图像，图像中的高度为损失函数的值。</p><p><img src='image/20180106_090904.png' alt='' /></p><p>由于3-D图形不便于标注，所以将3-D图形转换为<strong>轮廓图(contour plot)</strong>，下面用轮廓图（下图中的右图）来作直观理解，其中相同颜色的一个圈代表着同一高度（同一 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-66-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="4.758ex" height="2.577ex" viewBox="0 -806.1 2048.7 1109.7" role="img" focusable="false" style="vertical-align: -0.705ex;"><defs><path stroke-width="1" id="E66-MJMATHI-4A" d="M447 625Q447 637 354 637H329Q323 642 323 645T325 664Q329 677 335 683H352Q393 681 498 681Q541 681 568 681T605 682T619 682Q633 682 633 672Q633 670 630 658Q626 642 623 640T604 637Q552 637 545 623Q541 610 483 376Q420 128 419 127Q397 64 333 21T195 -22Q137 -22 97 8T57 88Q57 130 80 152T132 174Q177 174 182 130Q182 98 164 80T123 56Q115 54 115 53T122 44Q148 15 197 15Q235 15 271 47T324 130Q328 142 387 380T447 625Z"></path><path stroke-width="1" id="E66-MJMAIN-28" d="M94 250Q94 319 104 381T127 488T164 576T202 643T244 695T277 729T302 750H315H319Q333 750 333 741Q333 738 316 720T275 667T226 581T184 443T167 250T184 58T225 -81T274 -167T316 -220T333 -241Q333 -250 318 -250H315H302L274 -226Q180 -141 137 -14T94 250Z"></path><path stroke-width="1" id="E66-MJMATHI-3B8" d="M35 200Q35 302 74 415T180 610T319 704Q320 704 327 704T339 705Q393 701 423 656Q462 596 462 495Q462 380 417 261T302 66T168 -10H161Q125 -10 99 10T60 63T41 130T35 200ZM383 566Q383 668 330 668Q294 668 260 623T204 521T170 421T157 371Q206 370 254 370L351 371Q352 372 359 404T375 484T383 566ZM113 132Q113 26 166 26Q181 26 198 36T239 74T287 161T335 307L340 324H145Q145 321 136 286T120 208T113 132Z"></path><path stroke-width="1" id="E66-MJMAIN-29" d="M60 749L64 750Q69 750 74 750H86L114 726Q208 641 251 514T294 250Q294 182 284 119T261 12T224 -76T186 -143T145 -194T113 -227T90 -246Q87 -249 86 -250H74Q66 -250 63 -250T58 -247T55 -238Q56 -237 66 -225Q221 -64 221 250T66 725Q56 737 55 738Q55 746 60 749Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E66-MJMATHI-4A" x="0" y="0"></use><g transform="translate(800,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E66-MJMAIN-28" x="0" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E66-MJMATHI-3B8" x="389" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E66-MJMAIN-29" x="859" y="0"></use></g></g></svg></span><script type="math/tex" id="MathJax-Element-66">J\left(\theta\right)</script> 值）。</p><p><span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-48-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="16.17ex" height="2.461ex" viewBox="0 -806.1 6962.1 1059.4" role="img" focusable="false" style="vertical-align: -0.588ex;"><defs><path stroke-width="1" id="E48-MJMATHI-3B8" d="M35 200Q35 302 74 415T180 610T319 704Q320 704 327 704T339 705Q393 701 423 656Q462 596 462 495Q462 380 417 261T302 66T168 -10H161Q125 -10 99 10T60 63T41 130T35 200ZM383 566Q383 668 330 668Q294 668 260 623T204 521T170 421T157 371Q206 370 254 370L351 371Q352 372 359 404T375 484T383 566ZM113 132Q113 26 166 26Q181 26 198 36T239 74T287 161T335 307L340 324H145Q145 321 136 286T120 208T113 132Z"></path><path stroke-width="1" id="E48-MJMAIN-30" d="M96 585Q152 666 249 666Q297 666 345 640T423 548Q460 465 460 320Q460 165 417 83Q397 41 362 16T301 -15T250 -22Q224 -22 198 -16T137 16T82 83Q39 165 39 320Q39 494 96 585ZM321 597Q291 629 250 629Q208 629 178 597Q153 571 145 525T137 333Q137 175 145 125T181 46Q209 16 250 16Q290 16 318 46Q347 76 354 130T362 333Q362 478 354 524T321 597Z"></path><path stroke-width="1" id="E48-MJMAIN-3D" d="M56 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transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E48-MJMAIN-31" x="663" y="-213"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E48-MJMAIN-3D" x="5405" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E48-MJMAIN-30" x="6461" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-48">\theta_0 = 360, \theta_1 =0</script> 时：</p><p><img src='image/0f38a99c8ceb8aa5b90a5f12136fdf43.png' alt='' /></p><p>大概在 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-49-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="19.142ex" height="2.461ex" viewBox="0 -806.1 8241.6 1059.4" role="img" focusable="false" style="vertical-align: -0.588ex;"><defs><path stroke-width="1" id="E49-MJMATHI-3B8" d="M35 200Q35 302 74 415T180 610T319 704Q320 704 327 704T339 705Q393 701 423 656Q462 596 462 495Q462 380 417 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336Q478 285 440 178T402 50Q403 36 407 31T422 26Q450 26 474 56T513 138Q516 149 519 151T535 153Q555 153 555 145Q555 144 551 130Q535 71 500 33Q466 -10 419 -10H414Q367 -10 346 17T325 74Q325 90 361 192T398 345Q398 404 354 404H349Q266 404 205 306L198 293L164 158Q132 28 127 16Q114 -11 83 -11Q69 -11 59 -2T48 16Q48 30 121 320L195 616Q195 629 188 632T149 637H128Q122 643 122 645T124 664Q129 683 137 683Z"></path><path stroke-width="1" id="E50-MJMATHI-3B8" d="M35 200Q35 302 74 415T180 610T319 704Q320 704 327 704T339 705Q393 701 423 656Q462 596 462 495Q462 380 417 261T302 66T168 -10H161Q125 -10 99 10T60 63T41 130T35 200ZM383 566Q383 668 330 668Q294 668 260 623T204 521T170 421T157 371Q206 370 254 370L351 371Q352 372 359 404T375 484T383 566ZM113 132Q113 26 166 26Q181 26 198 36T239 74T287 161T335 307L340 324H145Q145 321 136 286T120 208T113 132Z"></path><path stroke-width="1" id="E50-MJMAIN-28" d="M94 250Q94 319 104 381T127 488T164 576T202 643T244 695T277 729T302 750H315H319Q333 750 333 741Q333 738 316 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'></a>2.5 梯度下降(Gradient Descent)</h2><p>在特征量很大的情况下，即便是借用计算机来生成图像，人工的方法也很难读出 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-66-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="4.758ex" height="2.577ex" viewBox="0 -806.1 2048.7 1109.7" role="img" focusable="false" style="vertical-align: -0.705ex;"><defs><path stroke-width="1" id="E66-MJMATHI-4A" d="M447 625Q447 637 354 637H329Q323 642 323 645T325 664Q329 677 335 683H352Q393 681 498 681Q541 681 568 681T605 682T619 682Q633 682 633 672Q633 670 630 658Q626 642 623 640T604 637Q552 637 545 623Q541 610 483 376Q420 128 419 127Q397 64 333 21T195 -22Q137 -22 97 8T57 88Q57 130 80 152T132 174Q177 174 182 130Q182 98 164 80T123 56Q115 54 115 53T122 44Q148 15 197 15Q235 15 271 47T324 130Q328 142 387 380T447 625Z"></path><path stroke-width="1" id="E66-MJMAIN-28" d="M94 250Q94 319 104 381T127 488T164 576T202 643T244 695T277 729T302 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minimum)</strong>，由于下降的情况只考虑当前参数组合周围的情况，所以无法确定当前的局部最小值是否就是<strong>全局最小值(global minimum)</strong>，不同的初始参数组合，可能会产生不同的局部最小值。</p><p>下图根据不同的起始点，产生了两个不同的局部最小值。</p><p><img src='image/db48c81304317847870d486ba5bb2015.jpg' alt='' /></p><p>视频中举了下山的例子，即我们在山顶上的某个位置，为了下山，就不断地看一下周围<strong>下一步往哪走</strong>下山比较快，然后就<strong>迈出那一步</strong>，一直重复，直到我们到达山下的某一处<strong>陆地</strong>。</p><p>梯度下降公式：</p><p><span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-74-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="28.85ex" height="12.149ex" viewBox="0 -2866.8 12421.4 5231" role="img" focusable="false" style="vertical-align: -5.491ex;"><defs><path stroke-width="1" id="E74-MJMAIN-72" d="M36 46H50Q89 46 97 60V68Q97 77 97 91T98 122T98 161T98 203Q98 234 98 269T98 328L97 351Q94 370 83 376T38 385H20V408Q20 431 22 431L32 432Q42 433 60 434T96 436Q112 437 131 438T160 441T171 442H174V373Q213 441 271 441H277Q322 441 343 419T364 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type="math/tex" id="MathJax-Element-59">\frac{\partial }{\partial {{\theta }_{j}}}J\left( \theta_0, \theta_1  \right)</script>: <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-60-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="8.991ex" height="2.577ex" viewBox="0 -806.1 3871.1 1109.7" role="img" focusable="false" style="vertical-align: -0.705ex;"><defs><path stroke-width="1" id="E60-MJMATHI-4A" d="M447 625Q447 637 354 637H329Q323 642 323 645T325 664Q329 677 335 683H352Q393 681 498 681Q541 681 568 681T605 682T619 682Q633 682 633 672Q633 670 630 658Q626 642 623 640T604 637Q552 637 545 623Q541 610 483 376Q420 128 419 127Q397 64 333 21T195 -22Q137 -22 97 8T57 88Q57 130 80 152T132 174Q177 174 182 130Q182 98 164 80T123 56Q115 54 115 53T122 44Q148 15 197 15Q235 15 271 47T324 130Q328 142 387 380T447 625Z"></path><path stroke-width="1" id="E60-MJMAIN-28" d="M94 250Q94 319 104 381T127 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type="math/tex" id="MathJax-Element-60">J\left( \theta_0, \theta_1 \right)</script> 的偏导</p></blockquote><p>公式中，学习速率决定了参数值变化的速率即”<strong>走多少距离</strong>“，而偏导这部分决定了下降的方向即”<strong>下一步往哪里</strong>“走（当然实际上的走多少距离是由偏导值给出的，学习速率起到调整后决定的作用），收敛处的局部最小值又叫做极小值，即”<strong>陆地</strong>“。</p><p><img src='image/20180106_101659.png' alt='' /></p><p>注意，在计算时要<strong>批量更新 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-69-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="1.09ex" height="2.11ex" viewBox="0 -806.1 469.5 908.7" role="img" focusable="false" style="vertical-align: -0.238ex;"><defs><path stroke-width="1" id="E69-MJMATHI-3B8" d="M35 200Q35 302 74 415T180 610T319 704Q320 704 327 704T339 705Q393 701 423 656Q462 596 462 495Q462 380 417 261T302 66T168 -10H161Q125 -10 99 10T60 63T41 130T35 200ZM383 566Q383 668 330 668Q294 668 260 623T204 521T170 421T157 371Q206 370 254 370L351 371Q352 372 359 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y="-213"></use></g></svg></span><script type="math/tex" id="MathJax-Element-68">\theta_1</script> 会<strong>向左边移动</strong>。这样不断重复，直到收敛（达到局部最小值，即斜率为0）。</p><p>初始 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-69-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="1.09ex" height="2.11ex" viewBox="0 -806.1 469.5 908.7" role="img" focusable="false" style="vertical-align: -0.238ex;"><defs><path stroke-width="1" id="E69-MJMATHI-3B8" d="M35 200Q35 302 74 415T180 610T319 704Q320 704 327 704T339 705Q393 701 423 656Q462 596 462 495Q462 380 417 261T302 66T168 -10H161Q125 -10 99 10T60 63T41 130T35 200ZM383 566Q383 668 330 668Q294 668 260 623T204 521T170 421T157 371Q206 370 254 370L351 371Q352 372 359 404T375 484T383 566ZM113 132Q113 26 166 26Q181 26 198 36T239 74T287 161T335 307L340 324H145Q145 321 136 286T120 208T113 132Z"></path></defs><g stroke="currentColor" fill="currentColor" 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xlink:href="#E71-MJMATHI-3B1" x="0" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-71">\alpha</script>，需要选取一个合适的值才能使得梯度下降算法运行良好。</p><ul><li><p>学习速率过小图示：</p><p><img src='image/20180106_190944.png' alt='' /></p><p>收敛的太慢，需要更多次的迭代。</p></li><li><p>学习速率过大图示：</p><p><img src='image/20180106_191023.png' alt='' /></p><p>可能越过最低点，甚至导致无法收敛。</p></li></ul><p><strong>学习速率只需选定即可</strong>，不需要在运行梯度下降算法的时候进行动态改变，随着斜率越来越接近于0，损失函数的变化幅度会越来越小，直到收敛到局部极小值。</p><p>如图，品红色点为初始点，损失函数随着迭代的进行，变化的幅度越来越小。</p><p><img src='image/20180106_191956.png' alt='' /></p><p><strong>最后，梯度下降不止可以用于线性回归中的损失函数，还通用于最小化其他的损失函数。</strong></p><h2><a name='header-n385' class='md-header-anchor '></a>2.7 线性回归中的梯度下降(Gradient Descent For Linear Regression)</h2><p>线性回归模型</p><ul><li><span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-72-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="17.039ex" height="2.577ex" 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xlink:href="#E83-MJMAIN-3D" x="345" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E83-MJMAIN-31" x="1124" y="0"></use></g><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E83-MJMATHI-6D" x="372" y="1344"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E83-MJSZ3-29" x="1885" y="-1"></use></g></svg></span><script type="math/tex" id="MathJax-Element-83">\left(\sum\limits_{i=1}^{m}\right)</script> 更新。</p><p>由于线性回归函数呈现<strong>碗状</strong>，且<strong>只有一个</strong>全局的最优值，所以函数<strong>一定总会</strong>收敛到全局最小值（学习速率不可过大）。同时，函数 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-84-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="1.471ex" height="1.994ex" viewBox="0 -755.9 633.5 858.4" role="img" focusable="false" style="vertical-align: -0.238ex;"><defs><path stroke-width="1" id="E84-MJMATHI-4A" d="M447 625Q447 637 354 637H329Q323 642 323 645T325 664Q329 677 335 683H352Q393 681 498 681Q541 681 568 681T605 682T619 682Q633 682 633 672Q633 670 630 658Q626 642 623 640T604 637Q552 637 545 623Q541 610 483 376Q420 128 419 127Q397 64 333 21T195 -22Q137 -22 97 8T57 88Q57 130 80 152T132 174Q177 174 182 130Q182 98 164 80T123 56Q115 54 115 53T122 44Q148 15 197 15Q235 15 271 47T324 130Q328 142 387 380T447 625Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E84-MJMATHI-4A" x="0" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-84">J</script> 被称为凸二次函数，而线性回归函数求解最小值问题属于<strong>凸函数优化问题</strong>。</p><p><img src='image/24e9420f16fdd758ccb7097788f879e7.png' alt='' /></p><p>另外，使用循环求解，代码较为冗余，后面会讲到如何使用<strong>向量化(Vectorization)</strong>来简化代码并优化计算，使梯度下降运行的更快更好。</p><h1><a name='header-n431' class='md-header-anchor '></a>3 Linear Algebra Review</h1><p>这部分，学过线性代数的可以复习一下，比较基础。笔记整理暂留。</p><h2><a name='header-n434' class='md-header-anchor '></a>3.1 Matrices and Vectors</h2><p>Octave/Matlab 代码:</p><pre class="md-fences md-end-block md-fences-with-lineno" lang="matlab" style="break-inside: unset;"> <div class="CodeMirror cm-s-inner CodeMirror-wrap"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 0px; left: 39px;"></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 35px; margin-bottom: 0px; border-right-width: 30px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre><div class="CodeMirror-linenumber CodeMirror-gutter-elt"><div>17</div></div></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation"><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: -35px; width: 35px;"></div><div class="CodeMirror-gutter-wrapper CodeMirror-activeline-gutter" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">1</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% The ; denotes we are going back to a new row.</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">2</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">A</span> = [<span class="cm-number">1</span>, <span class="cm-number">2</span>, <span class="cm-number">3</span>; <span class="cm-number">4</span>, <span class="cm-number">5</span>, <span class="cm-number">6</span>; <span class="cm-number">7</span>, <span class="cm-number">8</span>, <span class="cm-number">9</span>; <span class="cm-number">10</span>, <span class="cm-number">11</span>, <span class="cm-number">12</span>]</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">3</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">4</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% Initialize a vector </span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">5</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">v</span> = [<span class="cm-number">1</span>;<span class="cm-number">2</span>;<span class="cm-number">3</span>] </span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">6</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">7</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% Get the dimension of the matrix A where m = rows and n = columns</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">8</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">[<span class="cm-variable">m</span>,<span class="cm-variable">n</span>] = <span class="cm-builtin">size</span>(<span class="cm-variable">A</span>)</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">9</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">10</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% You could also store it this way</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">11</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">dim_A</span> = <span class="cm-builtin">size</span>(<span class="cm-variable">A</span>)</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">12</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">13</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% Get the dimension of the vector v </span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">14</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">dim_v</span> = <span class="cm-builtin">size</span>(<span class="cm-variable">v</span>)</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">15</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">16</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% Now let's index into the 2nd row 3rd column of matrix A</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">17</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">A_23</span> = <span class="cm-variable">A</span>(<span class="cm-number">2</span>,<span class="cm-number">3</span>)</span></pre></div></div></div></div></div></div><div style="position: absolute; height: 30px; width: 1px; border-bottom: 0px solid transparent; top: 403px;"></div><div class="CodeMirror-gutters" style="height: 433px;"><div class="CodeMirror-gutter CodeMirror-linenumbers" style="width: 34px;"></div></div></div></div></pre><p>执行结果:</p><pre class="md-fences md-end-block md-fences-with-lineno" lang="" style="break-inside: unset;"> <div class="CodeMirror cm-s-inner CodeMirror-wrap"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 0px; left: 39px;"></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 35px; margin-bottom: 0px; border-right-width: 30px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre><div class="CodeMirror-linenumber CodeMirror-gutter-elt"><div>24</div></div></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation"><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: -35px; width: 35px;"></div><div class="CodeMirror-gutter-wrapper CodeMirror-activeline-gutter" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">1</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">A =</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">2</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">3</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  1 &nbsp;  2 &nbsp;  3</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">4</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  4 &nbsp;  5 &nbsp;  6</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">5</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  7 &nbsp;  8 &nbsp;  9</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">6</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 10 &nbsp; 11 &nbsp; 12</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">7</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">8</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">v =</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">9</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">10</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 1</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">11</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 2</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">12</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 3</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">13</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">14</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">m =  4</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">15</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">n =  3</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">16</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">dim_A =</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">17</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">18</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 4 &nbsp; 3</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">19</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">20</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">dim_v =</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">21</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">22</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 3 &nbsp; 1</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">23</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">24</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">A_23 =  6</span></pre></div></div></div></div></div></div><div style="position: absolute; height: 30px; width: 1px; border-bottom: 0px solid transparent; top: 538px;"></div><div class="CodeMirror-gutters" style="height: 568px;"><div class="CodeMirror-gutter CodeMirror-linenumbers" style="width: 34px;"></div></div></div></div></pre><h2><a name='header-n441' class='md-header-anchor '></a>3.2 Addition and Scalar Multiplication</h2><p>Octave/Matlab 代码:</p><pre class="md-fences md-end-block md-fences-with-lineno" lang="matlab" style="break-inside: unset;"> <div class="CodeMirror cm-s-inner CodeMirror-wrap"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 0px; left: 39px;"></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 35px; margin-bottom: 0px; border-right-width: 30px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre><div class="CodeMirror-linenumber CodeMirror-gutter-elt"><div>21</div></div></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation"><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: -35px; width: 35px;"></div><div class="CodeMirror-gutter-wrapper CodeMirror-activeline-gutter" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">1</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% Initialize matrix A and B </span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">2</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">A</span> = [<span class="cm-number">1</span>, <span class="cm-number">2</span>, <span class="cm-number">4</span>; <span class="cm-number">5</span>, <span class="cm-number">3</span>, <span class="cm-number">2</span>]</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">3</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">B</span> = [<span class="cm-number">1</span>, <span class="cm-number">3</span>, <span class="cm-number">4</span>; <span class="cm-number">1</span>, <span class="cm-number">1</span>, <span class="cm-number">1</span>]</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">4</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">5</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% Initialize constant s </span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">6</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">s</span> = <span class="cm-number">2</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">7</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">8</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% See how element-wise addition works</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">9</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">add_AB</span> = <span class="cm-variable">A</span> <span class="cm-operator">+</span> <span class="cm-variable">B</span> </span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">10</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">11</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% See how element-wise subtraction works</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">12</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">sub_AB</span> = <span class="cm-variable">A</span> <span class="cm-operator">-</span> <span class="cm-variable">B</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">13</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">14</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% See how scalar multiplication works</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">15</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">mult_As</span> = <span class="cm-variable">A</span> <span class="cm-operator">*</span> <span class="cm-variable">s</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">16</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">17</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% Divide A by s</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">18</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">div_As</span> = <span class="cm-variable">A</span> <span class="cm-operator">/</span> <span class="cm-variable">s</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">19</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">20</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% What happens if we have a Matrix + scalar?</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">21</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">add_As</span> = <span class="cm-variable">A</span> <span class="cm-operator">+</span> <span class="cm-variable">s</span></span></pre></div></div></div></div></div></div><div style="position: absolute; height: 30px; width: 1px; border-bottom: 0px solid transparent; top: 470px;"></div><div class="CodeMirror-gutters" style="height: 500px;"><div class="CodeMirror-gutter CodeMirror-linenumbers" style="width: 34px;"></div></div></div></div></pre><p>执行结果:</p><pre class="md-fences md-end-block md-fences-with-lineno" lang="" style="break-inside: unset;"> <div class="CodeMirror cm-s-inner CodeMirror-wrap"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 0px; left: 39px;"></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 35px; margin-bottom: 0px; border-right-width: 30px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre><div class="CodeMirror-linenumber CodeMirror-gutter-elt"><div>35</div></div></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation"><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: -35px; width: 35px;"></div><div class="CodeMirror-gutter-wrapper CodeMirror-activeline-gutter" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">1</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">A =</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">2</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">3</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 1 &nbsp; 2 &nbsp; 4</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">4</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 5 &nbsp; 3 &nbsp; 2</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">5</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">6</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">B =</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">7</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">8</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 1 &nbsp; 3 &nbsp; 4</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">9</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 1 &nbsp; 1 &nbsp; 1</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">10</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">11</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">s =  2</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">12</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">add_AB =</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">13</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">14</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 2 &nbsp; 5 &nbsp; 8</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">15</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 6 &nbsp; 4 &nbsp; 3</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">16</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">17</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">sub_AB =</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">18</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">19</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 0  -1 &nbsp; 0</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">20</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 4 &nbsp; 2 &nbsp; 1</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">21</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">22</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">mult_As =</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">23</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">24</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  2 &nbsp;  4 &nbsp;  8</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">25</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 10 &nbsp;  6 &nbsp;  4</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">26</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">27</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">div_As =</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">28</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">29</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 0.50000 &nbsp; 1.00000 &nbsp; 2.00000</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">30</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 2.50000 &nbsp; 1.50000 &nbsp; 1.00000</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">31</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">32</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">add_As =</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">33</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">34</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 3 &nbsp; 4 &nbsp; 6</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">35</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 7 &nbsp; 5 &nbsp; 4</span></pre></div></div></div></div></div></div><div style="position: absolute; height: 30px; width: 1px; border-bottom: 0px solid transparent; top: 784px;"></div><div class="CodeMirror-gutters" style="height: 814px;"><div class="CodeMirror-gutter CodeMirror-linenumbers" style="width: 34px;"></div></div></div></div></pre><h2><a name='header-n448' class='md-header-anchor '></a>3.3 Matrix Vector Multiplication</h2><p>Octave/Matlab 代码:</p><pre class="md-fences md-end-block md-fences-with-lineno" lang="matlab"> <div class="CodeMirror cm-s-inner CodeMirror-wrap"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 0px; left: 39px;"></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 35px; margin-bottom: 0px; border-right-width: 30px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre><div class="CodeMirror-linenumber CodeMirror-gutter-elt"><div>10</div></div></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation"><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: -35px; width: 35px;"></div><div class="CodeMirror-gutter-wrapper CodeMirror-activeline-gutter" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">1</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% Initialize matrix A </span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">2</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">A</span> = [<span class="cm-number">1</span>, <span class="cm-number">2</span>, <span class="cm-number">3</span>; <span class="cm-number">4</span>, <span class="cm-number">5</span>, <span class="cm-number">6</span>;<span class="cm-number">7</span>, <span class="cm-number">8</span>, <span class="cm-number">9</span>] </span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">3</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">4</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% Initialize vector v </span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">5</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">v</span> = [<span class="cm-number">1</span>; <span class="cm-number">1</span>; <span class="cm-number">1</span>] </span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">6</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">7</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% Multiply A * v</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">8</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">Av</span> = <span class="cm-variable">A</span> <span class="cm-operator">*</span> <span class="cm-variable">v</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">9</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">10</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div></div></div></div></div></div><div style="position: absolute; height: 30px; width: 1px; border-bottom: 0px solid transparent; top: 224px;"></div><div class="CodeMirror-gutters" style="height: 254px;"><div class="CodeMirror-gutter CodeMirror-linenumbers" style="width: 34px;"></div></div></div></div></pre><p>执行结果:</p><pre class="md-fences md-end-block md-fences-with-lineno" lang="" style="break-inside: unset;"> <div class="CodeMirror cm-s-inner CodeMirror-wrap"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 0px; left: 39px;"></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 35px; margin-bottom: 0px; border-right-width: 30px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre><div class="CodeMirror-linenumber CodeMirror-gutter-elt"><div>18</div></div></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation"><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: -35px; width: 35px;"></div><div class="CodeMirror-gutter-wrapper CodeMirror-activeline-gutter" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">1</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">A =</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">2</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">3</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 1 &nbsp; 2 &nbsp; 3</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">4</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 4 &nbsp; 5 &nbsp; 6</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">5</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 7 &nbsp; 8 &nbsp; 9</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">6</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">7</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">v =</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">8</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">9</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 1</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">10</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 1</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">11</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 1</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">12</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">13</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">Av =</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">14</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">15</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  6</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">16</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 15</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">17</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 24</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">18</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div></div></div></div></div></div><div style="position: absolute; height: 30px; width: 1px; border-bottom: 0px solid transparent; top: 403px;"></div><div class="CodeMirror-gutters" style="height: 433px;"><div class="CodeMirror-gutter CodeMirror-linenumbers" style="width: 34px;"></div></div></div></div></pre><h2><a name='header-n455' class='md-header-anchor '></a>3.4 Matrix Matrix Multiplication</h2><p>Octave/Matlab 代码:</p><pre class="md-fences md-end-block md-fences-with-lineno" lang="matlab"> <div class="CodeMirror cm-s-inner CodeMirror-wrap"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 0px; left: 39px;"></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 35px; margin-bottom: 0px; border-right-width: 30px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre><div class="CodeMirror-linenumber CodeMirror-gutter-elt"><div>10</div></div></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation"><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: -35px; width: 35px;"></div><div class="CodeMirror-gutter-wrapper CodeMirror-activeline-gutter" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">1</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% Initialize a 3 by 2 matrix </span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">2</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">A</span> = [<span class="cm-number">1</span>, <span class="cm-number">2</span>; <span class="cm-number">3</span>, <span class="cm-number">4</span>;<span class="cm-number">5</span>, <span class="cm-number">6</span>]</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">3</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">4</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% Initialize a 2 by 1 matrix </span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">5</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">B</span> = [<span class="cm-number">1</span>; <span class="cm-number">2</span>] </span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">6</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">7</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% We expect a resulting matrix of (3 by 2)*(2 by 1) = (3 by 1) </span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">8</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">mult_AB</span> = <span class="cm-variable">A</span><span class="cm-operator">*</span><span class="cm-variable">B</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">9</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">10</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% Make sure you understand why we got that result</span></span></pre></div></div></div></div></div></div><div style="position: absolute; height: 30px; width: 1px; border-bottom: 0px solid transparent; top: 224px;"></div><div class="CodeMirror-gutters" style="height: 254px;"><div class="CodeMirror-gutter CodeMirror-linenumbers" style="width: 34px;"></div></div></div></div></pre><p>执行结果:</p><pre class="md-fences md-end-block md-fences-with-lineno" lang="" style="break-inside: unset;"> <div class="CodeMirror cm-s-inner CodeMirror-wrap"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 0px; left: 39px;"></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 35px; margin-bottom: 0px; border-right-width: 30px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre><div class="CodeMirror-linenumber CodeMirror-gutter-elt"><div>17</div></div></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation"><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: -35px; width: 35px;"></div><div class="CodeMirror-gutter-wrapper CodeMirror-activeline-gutter" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">1</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">A =</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">2</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">3</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 1 &nbsp; 2</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">4</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 3 &nbsp; 4</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">5</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 5 &nbsp; 6</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">6</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">7</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">B =</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">8</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">9</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 1</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">10</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 2</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">11</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">12</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">mult_AB =</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">13</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">14</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  5</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">15</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 11</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">16</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 17</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">17</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div></div></div></div></div></div><div style="position: absolute; height: 30px; width: 1px; border-bottom: 0px solid transparent; top: 381px;"></div><div class="CodeMirror-gutters" style="height: 411px;"><div class="CodeMirror-gutter CodeMirror-linenumbers" style="width: 34px;"></div></div></div></div></pre><h2><a name='header-n462' class='md-header-anchor '></a>3.5 Matrix Multiplication Properties</h2><p>Octave/Matlab 代码:</p><pre class="md-fences md-end-block md-fences-with-lineno" lang="matlab" style="break-inside: unset;"> <div class="CodeMirror cm-s-inner CodeMirror-wrap"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 0px; left: 39px;"></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 35px; margin-bottom: 0px; border-right-width: 30px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre><div class="CodeMirror-linenumber CodeMirror-gutter-elt"><div>22</div></div></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation"><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: -35px; width: 35px;"></div><div class="CodeMirror-gutter-wrapper CodeMirror-activeline-gutter" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">1</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% Initialize random matrices A and B </span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">2</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">A</span> = [<span class="cm-number">1</span>,<span class="cm-number">2</span>;<span class="cm-number">4</span>,<span class="cm-number">5</span>]</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">3</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">B</span> = [<span class="cm-number">1</span>,<span class="cm-number">1</span>;<span class="cm-number">0</span>,<span class="cm-number">2</span>]</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">4</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">5</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% Initialize a 2 by 2 identity matrix</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">6</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">I</span> = <span class="cm-builtin">eye</span>(<span class="cm-number">2</span>)</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">7</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">8</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% The above notation is the same as I = [1,0;0,1]</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">9</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">10</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% What happens when we multiply I*A ? </span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">11</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">IA</span> = <span class="cm-variable">I</span><span class="cm-operator">*</span><span class="cm-variable">A</span> </span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">12</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">13</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% How about A*I ? </span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">14</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">AI</span> = <span class="cm-variable">A</span><span class="cm-operator">*</span><span class="cm-variable">I</span> </span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">15</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">16</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% Compute A*B </span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">17</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">AB</span> = <span class="cm-variable">A</span><span class="cm-operator">*</span><span class="cm-variable">B</span> </span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">18</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">19</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% Is it equal to B*A? </span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">20</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">BA</span> = <span class="cm-variable">B</span><span class="cm-operator">*</span><span class="cm-variable">A</span> </span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">21</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">22</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% Note that IA = AI but AB != BA</span></span></pre></div></div></div></div></div></div><div style="position: absolute; height: 30px; width: 1px; border-bottom: 0px solid transparent; top: 493px;"></div><div class="CodeMirror-gutters" style="height: 523px;"><div class="CodeMirror-gutter CodeMirror-linenumbers" style="width: 34px;"></div></div></div></div></pre><p>执行结果:</p><pre class="md-fences md-end-block md-fences-with-lineno" lang="" style="break-inside: unset;"> <div class="CodeMirror cm-s-inner CodeMirror-wrap"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 0px; left: 39px;"></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 35px; margin-bottom: 0px; border-right-width: 30px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre><div class="CodeMirror-linenumber CodeMirror-gutter-elt"><div>36</div></div></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation"><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: -35px; width: 35px;"></div><div class="CodeMirror-gutter-wrapper CodeMirror-activeline-gutter" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">1</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">A =</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">2</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">3</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 1 &nbsp; 2</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">4</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 4 &nbsp; 5</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">5</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">6</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">B =</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">7</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">8</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 1 &nbsp; 1</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">9</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 0 &nbsp; 2</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">10</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">11</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">I =</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">12</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">13</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">Diagonal Matrix</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">14</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">15</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 1 &nbsp; 0</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">16</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 0 &nbsp; 1</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">17</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">18</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">IA =</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">19</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">20</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 1 &nbsp; 2</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">21</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 4 &nbsp; 5</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">22</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">23</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">AI =</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">24</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">25</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 1 &nbsp; 2</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">26</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 4 &nbsp; 5</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">27</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">28</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">AB =</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">29</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">30</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  1 &nbsp;  5</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">31</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  4 &nbsp; 14</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">32</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">33</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">BA =</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">34</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">35</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  5 &nbsp;  7</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">36</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  8 &nbsp; 10</span></pre></div></div></div></div></div></div><div style="position: absolute; height: 30px; width: 1px; border-bottom: 0px solid transparent; top: 806px;"></div><div class="CodeMirror-gutters" style="height: 836px;"><div class="CodeMirror-gutter CodeMirror-linenumbers" style="width: 34px;"></div></div></div></div></pre><h2><a name='header-n469' class='md-header-anchor '></a>3.6 Inverse and Transpose</h2><p>Octave/Matlab 代码:</p><pre class="md-fences md-end-block md-fences-with-lineno" lang="matlab"> <div class="CodeMirror cm-s-inner CodeMirror-wrap"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 0px; left: 39px;"></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 35px; margin-bottom: 0px; border-right-width: 30px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre><div class="CodeMirror-linenumber CodeMirror-gutter-elt"><div>13</div></div></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation"><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: -35px; width: 35px;"></div><div class="CodeMirror-gutter-wrapper CodeMirror-activeline-gutter" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">1</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% Initialize matrix A </span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">2</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">A</span> = [<span class="cm-number">1</span>,<span class="cm-number">2</span>,<span class="cm-number">0</span>;<span class="cm-number">0</span>,<span class="cm-number">5</span>,<span class="cm-number">6</span>;<span class="cm-number">7</span>,<span class="cm-number">0</span>,<span class="cm-number">9</span>]</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">3</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">4</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% Transpose A </span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">5</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">A_trans</span> = <span class="cm-variable">A</span><span class="cm-operator">'</span> </span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">6</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">7</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% Take the inverse of A </span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">8</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">A_inv</span> = <span class="cm-builtin">inv</span>(<span class="cm-variable">A</span>)</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">9</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">10</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% What is A^(-1)*A? </span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">11</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">A_invA</span> = <span class="cm-builtin">inv</span>(<span class="cm-variable">A</span>)<span class="cm-operator">*</span><span class="cm-variable">A</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">12</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">13</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div></div></div></div></div></div><div style="position: absolute; height: 30px; width: 1px; border-bottom: 0px solid transparent; top: 291px;"></div><div class="CodeMirror-gutters" style="height: 321px;"><div class="CodeMirror-gutter CodeMirror-linenumbers" style="width: 34px;"></div></div></div></div></pre><p>执行结果:</p><pre class="md-fences md-end-block md-fences-with-lineno" lang="" style="break-inside: unset;"> <div class="CodeMirror cm-s-inner CodeMirror-wrap"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 0px; left: 39px;"></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 35px; margin-bottom: 0px; border-right-width: 30px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre><div class="CodeMirror-linenumber CodeMirror-gutter-elt"><div>24</div></div></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation"><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: -35px; width: 35px;"></div><div class="CodeMirror-gutter-wrapper CodeMirror-activeline-gutter" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">1</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">A =</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">2</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">3</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 1 &nbsp; 2 &nbsp; 0</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">4</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 0 &nbsp; 5 &nbsp; 6</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">5</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 7 &nbsp; 0 &nbsp; 9</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">6</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">7</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">A_trans =</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">8</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">9</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 1 &nbsp; 0 &nbsp; 7</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">10</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 2 &nbsp; 5 &nbsp; 0</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">11</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 0 &nbsp; 6 &nbsp; 9</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">12</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">13</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">A_inv =</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">14</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">15</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 0.348837  -0.139535 &nbsp; 0.093023</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">16</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 0.325581 &nbsp; 0.069767  -0.046512</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">17</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">  -0.271318 &nbsp; 0.108527 &nbsp; 0.038760</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">18</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">19</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">A_invA =</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">20</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">21</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 1.00000  -0.00000 &nbsp; 0.00000</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">22</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 0.00000 &nbsp; 1.00000  -0.00000</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">23</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">  -0.00000 &nbsp; 0.00000 &nbsp; 1.00000</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">24</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div></div></div></div></div></div><div style="position: absolute; height: 30px; width: 1px; border-bottom: 0px solid transparent; top: 538px;"></div><div class="CodeMirror-gutters" style="height: 568px;"><div class="CodeMirror-gutter CodeMirror-linenumbers" style="width: 34px;"></div></div></div></div></pre></div>
+另外，在做<strong>原型搭建</strong>的时候也应该先考虑使用类似于 Octave 这种便于计算的编程软件，当其已经可以工作后，才将模型移植到其他的高级编程语言中。</p><blockquote><p>注：Octave 与 MATLAB 语法相近，由于 MATLAB 为商业软件，课程中使用开源且免费的 Octave。</p><p>机器学习领域发展迅速，现在也可使用 Tensorflow 等开源机器学习框架编写机器学习代码，这些框架十分友好，易于编写及应用。</p></blockquote><h1><a name='header-n199' class='md-header-anchor '></a>2 单变量线性回归(Linear Regression with One Variable)</h1><h2><a name='header-n200' class='md-header-anchor '></a>2.1 模型表示(Model Representation)</h2><ol start='' ><li><p>房价预测训练集</p><figure><table><thead><tr><th>Size in <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-164-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="5.339ex" height="2.757ex" viewBox="0 -889.4 2298.9 1187.2" role="img" focusable="false" style="vertical-align: -0.692ex;"><defs><path stroke-width="1" id="E165-MJMATHI-66" d="M118 -162Q120 -162 124 -164T135 -167T147 -168Q160 -168 171 -155T187 -126Q197 -99 221 27T267 267T289 382V385H242Q195 385 192 387Q188 390 188 397L195 425Q197 430 203 430T250 431Q298 431 298 432Q298 434 307 482T319 540Q356 705 465 705Q502 703 526 683T550 630Q550 594 529 578T487 561Q443 561 443 603Q443 622 454 636T478 657L487 662Q471 668 457 668Q445 668 434 658T419 630Q412 601 403 552T387 469T380 433Q380 431 435 431Q480 431 487 430T498 424Q499 420 496 407T491 391Q489 386 482 386T428 385H372L349 263Q301 15 282 -47Q255 -132 212 -173Q175 -205 139 -205Q107 -205 81 -186T55 -132Q55 -95 76 -78T118 -61Q162 -61 162 -103Q162 -122 151 -136T127 -157L118 -162Z"></path><path stroke-width="1" id="E165-MJMATHI-65" d="M39 168Q39 225 58 272T107 350T174 402T244 433T307 442H310Q355 442 388 420T421 355Q421 265 310 237Q261 224 176 223Q139 223 138 221Q138 219 132 186T125 128Q125 81 146 54T209 26T302 45T394 111Q403 121 406 121Q410 121 419 112T429 98T420 82T390 55T344 24T281 -1T205 -11Q126 -11 83 42T39 168ZM373 353Q367 405 305 405Q272 405 244 391T199 357T170 316T154 280T149 261Q149 260 169 260Q282 260 327 284T373 353Z"></path><path stroke-width="1" id="E165-MJMATHI-74" d="M26 385Q19 392 19 395Q19 399 22 411T27 425Q29 430 36 430T87 431H140L159 511Q162 522 166 540T173 566T179 586T187 603T197 615T211 624T229 626Q247 625 254 615T261 596Q261 589 252 549T232 470L222 433Q222 431 272 431H323Q330 424 330 420Q330 398 317 385H210L174 240Q135 80 135 68Q135 26 162 26Q197 26 230 60T283 144Q285 150 288 151T303 153H307Q322 153 322 145Q322 142 319 133Q314 117 301 95T267 48T216 6T155 -11Q125 -11 98 4T59 56Q57 64 57 83V101L92 241Q127 382 128 383Q128 385 77 385H26Z"></path><path stroke-width="1" id="E165-MJMAIN-32" d="M109 429Q82 429 66 447T50 491Q50 562 103 614T235 666Q326 666 387 610T449 465Q449 422 429 383T381 315T301 241Q265 210 201 149L142 93L218 92Q375 92 385 97Q392 99 409 186V189H449V186Q448 183 436 95T421 3V0H50V19V31Q50 38 56 46T86 81Q115 113 136 137Q145 147 170 174T204 211T233 244T261 278T284 308T305 340T320 369T333 401T340 431T343 464Q343 527 309 573T212 619Q179 619 154 602T119 569T109 550Q109 549 114 549Q132 549 151 535T170 489Q170 464 154 447T109 429Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E165-MJMATHI-66" x="0" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E165-MJMATHI-65" x="550" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E165-MJMATHI-65" x="1017" y="0"></use><g transform="translate(1483,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E165-MJMATHI-74" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E165-MJMAIN-32" x="511" y="513"></use></g></g></svg></span><script type="math/tex" id="MathJax-Element-164">feet^2</script> (<span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-19-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="1.33ex" height="1.41ex" viewBox="0 -504.6 572.5 607.1" role="img" focusable="false" style="vertical-align: -0.238ex;"><defs><path stroke-width="1" id="E20-MJMATHI-78" d="M52 289Q59 331 106 386T222 442Q257 442 286 424T329 379Q371 442 430 442Q467 442 494 420T522 361Q522 332 508 314T481 292T458 288Q439 288 427 299T415 328Q415 374 465 391Q454 404 425 404Q412 404 406 402Q368 386 350 336Q290 115 290 78Q290 50 306 38T341 26Q378 26 414 59T463 140Q466 150 469 151T485 153H489Q504 153 504 145Q504 144 502 134Q486 77 440 33T333 -11Q263 -11 227 52Q186 -10 133 -10H127Q78 -10 57 16T35 71Q35 103 54 123T99 143Q142 143 142 101Q142 81 130 66T107 46T94 41L91 40Q91 39 97 36T113 29T132 26Q168 26 194 71Q203 87 217 139T245 247T261 313Q266 340 266 352Q266 380 251 392T217 404Q177 404 142 372T93 290Q91 281 88 280T72 278H58Q52 284 52 289Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E20-MJMATHI-78" x="0" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-19">x</script>)</th><th>Price ($) in 1000&#39;s(<span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-7-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="1.155ex" height="1.877ex" viewBox="0 -504.6 497.5 808.1" role="img" focusable="false" style="vertical-align: -0.705ex;"><defs><path stroke-width="1" id="E8-MJMATHI-79" d="M21 287Q21 301 36 335T84 406T158 442Q199 442 224 419T250 355Q248 336 247 334Q247 331 231 288T198 191T182 105Q182 62 196 45T238 27Q261 27 281 38T312 61T339 94Q339 95 344 114T358 173T377 247Q415 397 419 404Q432 431 462 431Q475 431 483 424T494 412T496 403Q496 390 447 193T391 -23Q363 -106 294 -155T156 -205Q111 -205 77 -183T43 -117Q43 -95 50 -80T69 -58T89 -48T106 -45Q150 -45 150 -87Q150 -107 138 -122T115 -142T102 -147L99 -148Q101 -153 118 -160T152 -167H160Q177 -167 186 -165Q219 -156 247 -127T290 -65T313 -9T321 21L315 17Q309 13 296 6T270 -6Q250 -11 231 -11Q185 -11 150 11T104 82Q103 89 103 113Q103 170 138 262T173 379Q173 380 173 381Q173 390 173 393T169 400T158 404H154Q131 404 112 385T82 344T65 302T57 280Q55 278 41 278H27Q21 284 21 287Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E8-MJMATHI-79" x="0" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-7">y</script>)</th></tr></thead><tbody><tr><td>2104</td><td>460</td></tr><tr><td>1416</td><td>232</td></tr><tr><td>1534</td><td>315</td></tr><tr><td>852</td><td>178</td></tr><tr><td>...</td><td>...</td></tr></tbody></table></figure></li></ol><p>房价预测训练集中，同时给出了输入 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-19-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="1.33ex" height="1.41ex" viewBox="0 -504.6 572.5 607.1" role="img" focusable="false" style="vertical-align: -0.238ex;"><defs><path stroke-width="1" id="E20-MJMATHI-78" d="M52 289Q59 331 106 386T222 442Q257 442 286 424T329 379Q371 442 430 442Q467 442 494 420T522 361Q522 332 508 314T481 292T458 288Q439 288 427 299T415 328Q415 374 465 391Q454 404 425 404Q412 404 406 402Q368 386 350 336Q290 115 290 78Q290 50 306 38T341 26Q378 26 414 59T463 140Q466 150 469 151T485 153H489Q504 153 504 145Q504 144 502 134Q486 77 440 33T333 -11Q263 -11 227 52Q186 -10 133 -10H127Q78 -10 57 16T35 71Q35 103 54 123T99 143Q142 143 142 101Q142 81 130 66T107 46T94 41L91 40Q91 39 97 36T113 29T132 26Q168 26 194 71Q203 87 217 139T245 247T261 313Q266 340 266 352Q266 380 251 392T217 404Q177 404 142 372T93 290Q91 281 88 280T72 278H58Q52 284 52 289Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 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。假设函数根据输入(房屋的面积)，给出预测结果输出(房屋的价格)，即是一个 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-165-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="7.367ex" height="1.994ex" viewBox="0 -755.9 3172.1 858.4" role="img" focusable="false" style="vertical-align: -0.238ex;"><defs><path stroke-width="1" id="E166-MJMATHI-58" d="M42 0H40Q26 0 26 11Q26 15 29 27Q33 41 36 43T55 46Q141 49 190 98Q200 108 306 224T411 342Q302 620 297 625Q288 636 234 637H206Q200 643 200 645T202 664Q206 677 212 683H226Q260 681 347 681Q380 681 408 681T453 682T473 682Q490 682 490 671Q490 670 488 658Q484 643 481 640T465 637Q434 634 411 620L488 426L541 485Q646 598 646 610Q646 628 622 635Q617 635 609 637Q594 637 594 648Q594 650 596 664Q600 677 606 683H618Q619 683 643 683T697 681T738 680Q828 680 837 683H845Q852 676 852 672Q850 647 840 637H824Q790 636 763 628T722 611T698 593L687 584Q687 585 592 480L505 384Q505 383 536 304T601 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fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E211-MJMATHI-68" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E211-MJMATHI-3B8" x="815" y="-219"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E211-MJMAIN-28" x="1008" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E211-MJMATHI-78" x="1397" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E211-MJMAIN-29" x="1970" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E211-MJMAIN-3D" x="2637" y="0"></use><g transform="translate(3694,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E211-MJMATHI-3B8" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E211-MJMAIN-30" x="663" y="-213"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" 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442Q467 442 494 420T522 361Q522 332 508 314T481 292T458 288Q439 288 427 299T415 328Q415 374 465 391Q454 404 425 404Q412 404 406 402Q368 386 350 336Q290 115 290 78Q290 50 306 38T341 26Q378 26 414 59T463 140Q466 150 469 151T485 153H489Q504 153 504 145Q504 144 502 134Q486 77 440 33T333 -11Q263 -11 227 52Q186 -10 133 -10H127Q78 -10 57 16T35 71Q35 103 54 123T99 143Q142 143 142 101Q142 81 130 66T107 46T94 41L91 40Q91 39 97 36T113 29T132 26Q168 26 194 71Q203 87 217 139T245 247T261 313Q266 340 266 352Q266 380 251 392T217 404Q177 404 142 372T93 290Q91 281 88 280T72 278H58Q52 284 52 289Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E20-MJMATHI-78" x="0" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-19">x</script>: 特征/输入变量。</p></blockquote><p>上式中，<span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-65-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="1.09ex" height="2.11ex" viewBox="0 -806.1 469.5 908.7" role="img" focusable="false" style="vertical-align: -0.238ex;"><defs><path stroke-width="1" id="E66-MJMATHI-3B8" d="M35 200Q35 302 74 415T180 610T319 704Q320 704 327 704T339 705Q393 701 423 656Q462 596 462 495Q462 380 417 261T302 66T168 -10H161Q125 -10 99 10T60 63T41 130T35 200ZM383 566Q383 668 330 668Q294 668 260 623T204 521T170 421T157 371Q206 370 254 370L351 371Q352 372 359 404T375 484T383 566ZM113 132Q113 26 166 26Q181 26 198 36T239 74T287 161T335 307L340 324H145Q145 321 136 286T120 208T113 132Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E66-MJMATHI-3B8" x="0" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-65">\theta</script> 为参数，<span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-65-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="1.09ex" height="2.11ex" viewBox="0 -806.1 469.5 908.7" role="img" focusable="false" style="vertical-align: -0.238ex;"><defs><path stroke-width="1" id="E66-MJMATHI-3B8" d="M35 200Q35 302 74 415T180 610T319 704Q320 704 327 704T339 705Q393 701 423 656Q462 596 462 495Q462 380 417 261T302 66T168 -10H161Q125 -10 99 10T60 63T41 130T35 200ZM383 566Q383 668 330 668Q294 668 260 623T204 521T170 421T157 371Q206 370 254 370L351 371Q352 372 359 404T375 484T383 566ZM113 132Q113 26 166 26Q181 26 198 36T239 74T287 161T335 307L340 324H145Q145 321 136 286T120 208T113 132Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E66-MJMATHI-3B8" x="0" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-65">\theta</script> 的变化才决定了输出结果，不同以往，这里的 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-19-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="1.33ex" height="1.41ex" viewBox="0 -504.6 572.5 607.1" role="img" focusable="false" style="vertical-align: -0.238ex;"><defs><path stroke-width="1" id="E20-MJMATHI-78" d="M52 289Q59 331 106 386T222 442Q257 442 286 424T329 379Q371 442 430 442Q467 442 494 420T522 361Q522 332 508 314T481 292T458 288Q439 288 427 299T415 328Q415 374 465 391Q454 404 425 404Q412 404 406 402Q368 386 350 336Q290 115 290 78Q290 50 306 38T341 26Q378 26 414 59T463 140Q466 150 469 151T485 153H489Q504 153 504 145Q504 144 502 134Q486 77 440 33T333 -11Q263 -11 227 52Q186 -10 133 -10H127Q78 -10 57 16T35 71Q35 103 54 123T99 143Q142 143 142 101Q142 81 130 66T107 46T94 41L91 40Q91 39 97 36T113 29T132 26Q168 26 194 71Q203 87 217 139T245 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668 330 668Q294 668 260 623T204 521T170 421T157 371Q206 370 254 370L351 371Q352 372 359 404T375 484T383 566ZM113 132Q113 26 166 26Q181 26 198 36T239 74T287 161T335 307L340 324H145Q145 321 136 286T120 208T113 132Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E66-MJMATHI-3B8" x="0" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-65">\theta</script> 以更好地拟合数据，成了求解该问题的最终问题。</p><p>单变量，即只有一个特征(如例子中房屋的面积这个特征)。</p><h2><a name='header-n243' class='md-header-anchor '></a>2.2 损失函数(Cost Function)</h2><p>我们的目的在于求解预测结果 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-108-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="1.339ex" height="1.994ex" viewBox="0 -755.9 576.5 858.4" role="img" focusable="false" style="vertical-align: 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id="MathJax-Element-170">\left(x^\left(i\right),y^\left(i\right)\right)</script>: 训练集中的第 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-99-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="0.802ex" height="1.994ex" viewBox="0 -755.9 345.5 858.4" role="img" focusable="false" style="vertical-align: -0.238ex;"><defs><path stroke-width="1" id="E100-MJMATHI-69" d="M184 600Q184 624 203 642T247 661Q265 661 277 649T290 619Q290 596 270 577T226 557Q211 557 198 567T184 600ZM21 287Q21 295 30 318T54 369T98 420T158 442Q197 442 223 419T250 357Q250 340 236 301T196 196T154 83Q149 61 149 51Q149 26 166 26Q175 26 185 29T208 43T235 78T260 137Q263 149 265 151T282 153Q302 153 302 143Q302 135 293 112T268 61T223 11T161 -11Q129 -11 102 10T74 74Q74 91 79 106T122 220Q160 321 166 341T173 380Q173 404 156 404H154Q124 404 99 371T61 287Q60 286 59 284T58 281T56 279T53 278T49 278T41 278H27Q21 284 21 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y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E173-MJMAIN-32" x="7836" y="675"></use></g></g></g></g></svg></span><script type="math/tex" id="MathJax-Element-172">J(\theta_0,\theta_1)=\dfrac{1}{2m}\displaystyle\sum_{i=1}^m\left(\hat{y}_{i}-y_{i} \right)^2=\dfrac{1}{2m}\displaystyle\sum_{i=1}^m\left(h_\theta(x_{i})-y_{i}\right)^2</script> </p><blockquote><p><span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-173-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="1.302ex" height="2.461ex" viewBox="0 -755.9 560.7 1059.4" role="img" focusable="false" style="vertical-align: -0.705ex;"><defs><path stroke-width="1" id="E174-MJMATHI-79" d="M21 287Q21 301 36 335T84 406T158 442Q199 442 224 419T250 355Q248 336 247 334Q247 331 231 288T198 191T182 105Q182 62 196 45T238 27Q261 27 281 38T312 61T339 94Q339 95 344 114T358 173T377 247Q415 397 419 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。</p></blockquote><p>讨论到这里，我们的问题就转化成了<strong>求解 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-176-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="8.991ex" height="2.757ex" viewBox="0 -840.1 3871.1 1187.2" role="img" focusable="false" style="vertical-align: -0.806ex;"><defs><path stroke-width="1" id="E177-MJMATHI-4A" d="M447 625Q447 637 354 637H329Q323 642 323 645T325 664Q329 677 335 683H352Q393 681 498 681Q541 681 568 681T605 682T619 682Q633 682 633 672Q633 670 630 658Q626 642 623 640T604 637Q552 637 545 623Q541 610 483 376Q420 128 419 127Q397 64 333 21T195 -22Q137 -22 97 8T57 88Q57 130 80 152T132 174Q177 174 182 130Q182 98 164 80T123 56Q115 54 115 53T122 44Q148 15 197 15Q235 15 271 47T324 130Q328 142 387 380T447 625Z"></path><path stroke-width="1" id="E177-MJMAIN-28" d="M94 250Q94 319 104 381T127 488T164 576T202 643T244 695T277 729T302 750H315H319Q333 750 333 741Q333 738 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id="MathJax-Element-176">J\left( \theta_0, \theta_1  \right)</script> 的最小值</strong>。</p><h2><a name='header-n270' class='md-header-anchor '></a>2.3 损失函数 - 直观理解1(Cost Function - Intuition I)</h2><p>根据上节视频，列出如下定义：</p><ul><li>假设函数(Hypothesis): <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-210-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="17.039ex" height="2.577ex" viewBox="0 -806.1 7336.3 1109.7" role="img" focusable="false" style="vertical-align: -0.705ex;"><defs><path stroke-width="1" id="E211-MJMATHI-68" d="M137 683Q138 683 209 688T282 694Q294 694 294 685Q294 674 258 534Q220 386 220 383Q220 381 227 388Q288 442 357 442Q411 442 444 415T478 336Q478 285 440 178T402 50Q403 36 407 31T422 26Q450 26 474 56T513 138Q516 149 519 151T535 153Q555 153 555 145Q555 144 551 130Q535 71 500 33Q466 -10 419 -10H414Q367 -10 346 17T325 74Q325 90 361 192T398 345Q398 404 354 404H349Q266 404 205 306L198 293L164 158Q132 28 127 16Q114 -11 83 -11Q69 -11 59 -2T48 16Q48 30 121 320L195 616Q195 629 188 632T149 637H128Q122 643 122 645T124 664Q129 683 137 683Z"></path><path stroke-width="1" id="E211-MJMATHI-3B8" d="M35 200Q35 302 74 415T180 610T319 704Q320 704 327 704T339 705Q393 701 423 656Q462 596 462 495Q462 380 417 261T302 66T168 -10H161Q125 -10 99 10T60 63T41 130T35 200ZM383 566Q383 668 330 668Q294 668 260 623T204 521T170 421T157 371Q206 370 254 370L351 371Q352 372 359 404T375 484T383 566ZM113 132Q113 26 166 26Q181 26 198 36T239 74T287 161T335 307L340 324H145Q145 321 136 286T120 208T113 132Z"></path><path stroke-width="1" id="E211-MJMAIN-28" d="M94 250Q94 319 104 381T127 488T164 576T202 643T244 695T277 729T302 750H315H319Q333 750 333 741Q333 738 316 720T275 667T226 581T184 443T167 250T184 58T225 -81T274 -167T316 -220T333 -241Q333 -250 318 -250H315H302L274 -226Q180 -141 137 -14T94 250Z"></path><path stroke-width="1" id="E211-MJMATHI-78" d="M52 289Q59 331 106 386T222 442Q257 442 286 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x="663" y="-213"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E211-MJMAIN-2B" x="4839" y="0"></use><g transform="translate(5840,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E211-MJMATHI-3B8" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E211-MJMAIN-31" x="663" y="-213"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E211-MJMATHI-78" x="6763" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-210">h_\theta(x)=\theta_0+\theta_1x</script></li><li>参数(Parameters): <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-190-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="5.323ex" height="2.461ex" viewBox="0 -806.1 2292 1059.4" role="img" focusable="false" style="vertical-align: -0.588ex;"><defs><path stroke-width="1" id="E191-MJMATHI-3B8" d="M35 200Q35 302 74 415T180 610T319 704Q320 704 327 704T339 705Q393 701 423 656Q462 596 462 495Q462 380 417 261T302 66T168 -10H161Q125 -10 99 10T60 63T41 130T35 200ZM383 566Q383 668 330 668Q294 668 260 623T204 521T170 421T157 371Q206 370 254 370L351 371Q352 372 359 404T375 484T383 566ZM113 132Q113 26 166 26Q181 26 198 36T239 74T287 161T335 307L340 324H145Q145 321 136 286T120 208T113 132Z"></path><path stroke-width="1" id="E191-MJMAIN-30" d="M96 585Q152 666 249 666Q297 666 345 640T423 548Q460 465 460 320Q460 165 417 83Q397 41 362 16T301 -15T250 -22Q224 -22 198 -16T137 16T82 83Q39 165 39 320Q39 494 96 585ZM321 597Q291 629 250 629Q208 629 178 597Q153 571 145 525T137 333Q137 175 145 125T181 46Q209 16 250 16Q290 16 318 46Q347 76 354 130T362 333Q362 478 354 524T321 597Z"></path><path stroke-width="1" id="E191-MJMAIN-2C" d="M78 35T78 60T94 103T137 121Q165 121 187 96T210 8Q210 -27 201 -60T180 -117T154 -158T130 -185T117 -194Q113 -194 104 -185T95 -172Q95 -168 106 -156T131 -126T157 -76T173 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xlink:href="#E191-MJMAIN-31" x="663" y="-213"></use></g></g></svg></span><script type="math/tex" id="MathJax-Element-190">\theta_0, \theta_1</script></li><li>损失函数(Cost Function): <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-211-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="36.164ex" height="5.846ex" viewBox="0 -1459.5 15570.8 2517" role="img" focusable="false" style="vertical-align: -2.456ex;"><defs><path stroke-width="1" id="E212-MJMATHI-4A" d="M447 625Q447 637 354 637H329Q323 642 323 645T325 664Q329 677 335 683H352Q393 681 498 681Q541 681 568 681T605 682T619 682Q633 682 633 672Q633 670 630 658Q626 642 623 640T604 637Q552 637 545 623Q541 610 483 376Q420 128 419 127Q397 64 333 21T195 -22Q137 -22 97 8T57 88Q57 130 80 152T132 174Q177 174 182 130Q182 98 164 80T123 56Q115 54 115 53T122 44Q148 15 197 15Q235 15 271 47T324 130Q328 142 387 380T447 625Z"></path><path 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xlink:href="#E212-MJMAIN-32" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E212-MJMATHI-6D" x="500" y="0"></use></g></g></g><g transform="translate(6706,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E212-MJSZ1-2211" x="46" y="0"></use><g transform="translate(0,-890)"><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E212-MJMATHI-69" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E212-MJMAIN-3D" x="345" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E212-MJMAIN-31" x="1124" y="0"></use></g><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E212-MJMATHI-6D" x="372" y="1344"></use></g><g transform="translate(8022,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E212-MJSZ1-28"></use><g transform="translate(458,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E212-MJMATHI-68" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E212-MJMATHI-3B8" x="815" y="-219"></use></g><g transform="translate(1633,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E212-MJSZ1-28"></use><g transform="translate(458,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E212-MJMATHI-78" x="0" y="0"></use><g transform="translate(572,362)"><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E212-MJMAIN-28" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E212-MJMATHI-69" x="389" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E212-MJMAIN-29" x="734" y="0"></use></g></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E212-MJSZ1-29" x="1926" y="-1"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E212-MJMAIN-2212" x="4240" y="0"></use><g transform="translate(5241,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E212-MJMATHI-79" x="0" y="0"></use><g transform="translate(499,362)"><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E212-MJMAIN-28" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E212-MJMATHI-69" x="389" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E212-MJMAIN-29" x="734" y="0"></use></g></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E212-MJSZ1-29" x="6636" y="-1"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E212-MJMAIN-32" x="10033" y="878"></use></g></g></svg></span><script type="math/tex" id="MathJax-Element-211"> J\left( \theta_0, \theta_1  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260 623T204 521T170 421T157 371Q206 370 254 370L351 371Q352 372 359 404T375 484T383 566ZM113 132Q113 26 166 26Q181 26 198 36T239 74T287 161T335 307L340 324H145Q145 321 136 286T120 208T113 132Z"></path><path stroke-width="1" id="E189-MJMAIN-30" d="M96 585Q152 666 249 666Q297 666 345 640T423 548Q460 465 460 320Q460 165 417 83Q397 41 362 16T301 -15T250 -22Q224 -22 198 -16T137 16T82 83Q39 165 39 320Q39 494 96 585ZM321 597Q291 629 250 629Q208 629 178 597Q153 571 145 525T137 333Q137 175 145 125T181 46Q209 16 250 16Q290 16 318 46Q347 76 354 130T362 333Q362 478 354 524T321 597Z"></path><path stroke-width="1" id="E189-MJMAIN-2C" d="M78 35T78 60T94 103T137 121Q165 121 187 96T210 8Q210 -27 201 -60T180 -117T154 -158T130 -185T117 -194Q113 -194 104 -185T95 -172Q95 -168 106 -156T131 -126T157 -76T173 -3V9L172 8Q170 7 167 6T161 3T152 1T140 0Q113 0 96 17Z"></path><path stroke-width="1" id="E189-MJMAIN-31" d="M213 578L200 573Q186 568 160 563T102 556H83V602H102Q149 604 189 617T245 641T273 663Q275 666 285 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y="0"></use><g transform="translate(389,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E189-MJMATHI-3B8" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E189-MJMAIN-30" x="663" y="-213"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E189-MJMAIN-2C" x="1312" y="0"></use><g transform="translate(1758,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E189-MJMATHI-3B8" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E189-MJMAIN-31" x="663" y="-213"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E189-MJMAIN-29" x="2681" y="0"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E189-MJMAIN-3D" x="4148" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E189-MJMAIN-30" x="5205" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-188">J\left(\theta_0, \theta_1 \right) = 0</script>，取得最小值，</strong>对应于左图青色直线，即函数 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-108-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="1.339ex" height="1.994ex" viewBox="0 -755.9 576.5 858.4" role="img" focusable="false" style="vertical-align: -0.238ex;"><defs><path stroke-width="1" id="E109-MJMATHI-68" d="M137 683Q138 683 209 688T282 694Q294 694 294 685Q294 674 258 534Q220 386 220 383Q220 381 227 388Q288 442 357 442Q411 442 444 415T478 336Q478 285 440 178T402 50Q403 36 407 31T422 26Q450 26 474 56T513 138Q516 149 519 151T535 153Q555 153 555 145Q555 144 551 130Q535 71 500 33Q466 -10 419 -10H414Q367 -10 346 17T325 74Q325 90 361 192T398 345Q398 404 354 404H349Q266 404 205 306L198 293L164 158Q132 28 127 16Q114 -11 83 -11Q69 -11 59 -2T48 16Q48 30 121 320L195 616Q195 629 188 632T149 637H128Q122 643 122 645T124 664Q129 683 137 683Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E109-MJMATHI-68" x="0" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-108">h</script> 拟合程度最好的情况。</p><h2><a name='header-n292' class='md-header-anchor '></a>2.4 损失函数 - 直观理解2(Cost Function - Intuition II)</h2><blockquote><p>注：该部分由于涉及到了多变量成像，可能较难理解，要求只需要理解上节内容即可，该节如果不能较好理解可跳过。</p></blockquote><p>给定数据集：</p><p><img src='image/20180106_091307.png' alt='' /></p><p>参数在 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-104-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="2.145ex" height="2.461ex" viewBox="0 -806.1 923.4 1059.4" role="img" focusable="false" style="vertical-align: -0.588ex;"><defs><path stroke-width="1" id="E105-MJMATHI-3B8" d="M35 200Q35 302 74 415T180 610T319 704Q320 704 327 704T339 705Q393 701 423 656Q462 596 462 495Q462 380 417 261T302 66T168 -10H161Q125 -10 99 10T60 63T41 130T35 200ZM383 566Q383 668 330 668Q294 668 260 623T204 521T170 421T157 371Q206 370 254 370L351 371Q352 372 359 404T375 484T383 566ZM113 132Q113 26 166 26Q181 26 198 36T239 74T287 161T335 307L340 324H145Q145 321 136 286T120 208T113 132Z"></path><path stroke-width="1" id="E105-MJMAIN-30" d="M96 585Q152 666 249 666Q297 666 345 640T423 548Q460 465 460 320Q460 165 417 83Q397 41 362 16T301 -15T250 -22Q224 -22 198 -16T137 16T82 83Q39 165 39 320Q39 494 96 585ZM321 597Q291 629 250 629Q208 629 178 597Q153 571 145 525T137 333Q137 175 145 125T181 46Q209 16 250 16Q290 16 318 46Q347 76 354 130T362 333Q362 478 354 524T321 597Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E105-MJMATHI-3B8" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E105-MJMAIN-30" x="663" y="-213"></use></g></svg></span><script type="math/tex" id="MathJax-Element-104">\theta_0</script> 不恒为 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-59-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="1.162ex" height="1.994ex" viewBox="0 -755.9 500.5 858.4" role="img" focusable="false" style="vertical-align: -0.238ex;"><defs><path stroke-width="1" id="E60-MJMAIN-30" d="M96 585Q152 666 249 666Q297 666 345 640T423 548Q460 465 460 320Q460 165 417 83Q397 41 362 16T301 -15T250 -22Q224 -22 198 -16T137 16T82 83Q39 165 39 320Q39 494 96 585ZM321 597Q291 629 250 629Q208 629 178 597Q153 571 145 525T137 333Q137 175 145 125T181 46Q209 16 250 16Q290 16 318 46Q347 76 354 130T362 333Q362 478 354 524T321 597Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E60-MJMAIN-30" x="0" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-59">0</script> 时损失函数 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-206-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="4.758ex" height="2.577ex" viewBox="0 -806.1 2048.7 1109.7" role="img" focusable="false" style="vertical-align: -0.705ex;"><defs><path stroke-width="1" id="E207-MJMATHI-4A" d="M447 625Q447 637 354 637H329Q323 642 323 645T325 664Q329 677 335 683H352Q393 681 498 681Q541 681 568 681T605 682T619 682Q633 682 633 672Q633 670 630 658Q626 642 623 640T604 637Q552 637 545 623Q541 610 483 376Q420 128 419 127Q397 64 333 21T195 -22Q137 -22 97 8T57 88Q57 130 80 152T132 174Q177 174 182 130Q182 98 164 80T123 56Q115 54 115 53T122 44Q148 15 197 15Q235 15 271 47T324 130Q328 142 387 380T447 625Z"></path><path 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725Q56 737 55 738Q55 746 60 749Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E207-MJMATHI-4A" x="0" y="0"></use><g transform="translate(800,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E207-MJMAIN-28" x="0" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E207-MJMATHI-3B8" x="389" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E207-MJMAIN-29" x="859" y="0"></use></g></g></svg></span><script type="math/tex" id="MathJax-Element-206">J\left(\theta\right)</script> 关于 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-190-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="5.323ex" height="2.461ex" viewBox="0 -806.1 2292 1059.4" role="img" focusable="false" style="vertical-align: -0.588ex;"><defs><path stroke-width="1" id="E191-MJMATHI-3B8" d="M35 200Q35 302 74 415T180 610T319 704Q320 704 327 704T339 705Q393 701 423 656Q462 596 462 495Q462 380 417 261T302 66T168 -10H161Q125 -10 99 10T60 63T41 130T35 200ZM383 566Q383 668 330 668Q294 668 260 623T204 521T170 421T157 371Q206 370 254 370L351 371Q352 372 359 404T375 484T383 566ZM113 132Q113 26 166 26Q181 26 198 36T239 74T287 161T335 307L340 324H145Q145 321 136 286T120 208T113 132Z"></path><path stroke-width="1" id="E191-MJMAIN-30" d="M96 585Q152 666 249 666Q297 666 345 640T423 548Q460 465 460 320Q460 165 417 83Q397 41 362 16T301 -15T250 -22Q224 -22 198 -16T137 16T82 83Q39 165 39 320Q39 494 96 585ZM321 597Q291 629 250 629Q208 629 178 597Q153 571 145 525T137 333Q137 175 145 125T181 46Q209 16 250 16Q290 16 318 46Q347 76 354 130T362 333Q362 478 354 524T321 597Z"></path><path stroke-width="1" id="E191-MJMAIN-2C" d="M78 35T78 60T94 103T137 121Q165 121 187 96T210 8Q210 -27 201 -60T180 -117T154 -158T130 -185T117 -194Q113 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transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E191-MJMAIN-31" x="663" y="-213"></use></g></g></svg></span><script type="math/tex" id="MathJax-Element-190">\theta_0, \theta_1</script> 的3-D图像，图像中的高度为损失函数的值。</p><p><img src='image/20180106_090904.png' alt='' /></p><p>由于3-D图形不便于标注，所以将3-D图形转换为<strong>轮廓图(contour plot)</strong>，下面用轮廓图（下图中的右图）来作直观理解，其中相同颜色的一个圈代表着同一高度（同一 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-206-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="4.758ex" height="2.577ex" viewBox="0 -806.1 2048.7 1109.7" role="img" focusable="false" style="vertical-align: -0.705ex;"><defs><path stroke-width="1" id="E207-MJMATHI-4A" d="M447 625Q447 637 354 637H329Q323 642 323 645T325 664Q329 677 335 683H352Q393 681 498 681Q541 681 568 681T605 682T619 682Q633 682 633 672Q633 670 630 658Q626 642 623 640T604 637Q552 637 545 623Q541 610 483 376Q420 128 419 127Q397 64 333 21T195 -22Q137 -22 97 8T57 88Q57 130 80 152T132 174Q177 174 182 130Q182 98 164 80T123 56Q115 54 115 53T122 44Q148 15 197 15Q235 15 271 47T324 130Q328 142 387 380T447 625Z"></path><path stroke-width="1" id="E207-MJMAIN-28" d="M94 250Q94 319 104 381T127 488T164 576T202 643T244 695T277 729T302 750H315H319Q333 750 333 741Q333 738 316 720T275 667T226 581T184 443T167 250T184 58T225 -81T274 -167T316 -220T333 -241Q333 -250 318 -250H315H302L274 -226Q180 -141 137 -14T94 250Z"></path><path stroke-width="1" id="E207-MJMATHI-3B8" d="M35 200Q35 302 74 415T180 610T319 704Q320 704 327 704T339 705Q393 701 423 656Q462 596 462 495Q462 380 417 261T302 66T168 -10H161Q125 -10 99 10T60 63T41 130T35 200ZM383 566Q383 668 330 668Q294 668 260 623T204 521T170 421T157 371Q206 370 254 370L351 371Q352 372 359 404T375 484T383 566ZM113 132Q113 26 166 26Q181 26 198 36T239 74T287 161T335 307L340 324H145Q145 321 136 286T120 208T113 132Z"></path><path stroke-width="1" id="E207-MJMAIN-29" d="M60 749L64 750Q69 750 74 750H86L114 726Q208 641 251 514T294 250Q294 182 284 119T261 12T224 -76T186 -143T145 -194T113 -227T90 -246Q87 -249 86 -250H74Q66 -250 63 -250T58 -247T55 -238Q56 -237 66 -225Q221 -64 221 250T66 725Q56 737 55 738Q55 746 60 749Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E207-MJMATHI-4A" x="0" y="0"></use><g transform="translate(800,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E207-MJMAIN-28" x="0" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E207-MJMATHI-3B8" x="389" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E207-MJMAIN-29" x="859" y="0"></use></g></g></svg></span><script type="math/tex" id="MathJax-Element-206">J\left(\theta\right)</script> 值）。</p><p><span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-192-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="16.17ex" height="2.461ex" viewBox="0 -806.1 6962.1 1059.4" role="img" focusable="false" style="vertical-align: -0.588ex;"><defs><path stroke-width="1" id="E193-MJMATHI-3B8" d="M35 200Q35 302 74 415T180 610T319 704Q320 704 327 704T339 705Q393 701 423 656Q462 596 462 495Q462 380 417 261T302 66T168 -10H161Q125 -10 99 10T60 63T41 130T35 200ZM383 566Q383 668 330 668Q294 668 260 623T204 521T170 421T157 371Q206 370 254 370L351 371Q352 372 359 404T375 484T383 566ZM113 132Q113 26 166 26Q181 26 198 36T239 74T287 161T335 307L340 324H145Q145 321 136 286T120 208T113 132Z"></path><path stroke-width="1" id="E193-MJMAIN-30" d="M96 585Q152 666 249 666Q297 666 345 640T423 548Q460 465 460 320Q460 165 417 83Q397 41 362 16T301 -15T250 -22Q224 -22 198 -16T137 16T82 83Q39 165 39 320Q39 494 96 585ZM321 597Q291 629 250 629Q208 629 178 597Q153 571 145 525T137 333Q137 175 145 125T181 46Q209 16 250 16Q290 16 318 46Q347 76 354 130T362 333Q362 478 354 524T321 597Z"></path><path stroke-width="1" id="E193-MJMAIN-3D" d="M56 347Q56 360 70 367H707Q722 359 722 347Q722 336 708 328L390 327H72Q56 332 56 347ZM56 153Q56 168 72 173H708Q722 163 722 153Q722 140 707 133H70Q56 140 56 153Z"></path><path stroke-width="1" id="E193-MJMAIN-33" d="M127 463Q100 463 85 480T69 524Q69 579 117 622T233 665Q268 665 277 664Q351 652 390 611T430 522Q430 470 396 421T302 350L299 348Q299 347 308 345T337 336T375 315Q457 262 457 175Q457 96 395 37T238 -22Q158 -22 100 21T42 130Q42 158 60 175T105 193Q133 193 151 175T169 130Q169 119 166 110T159 94T148 82T136 74T126 70T118 67L114 66Q165 21 238 21Q293 21 321 74Q338 107 338 175V195Q338 290 274 322Q259 328 213 329L171 330L168 332Q166 335 166 348Q166 366 174 366Q202 366 232 371Q266 376 294 413T322 525V533Q322 590 287 612Q265 626 240 626Q208 626 181 615T143 592T132 580H135Q138 579 143 578T153 573T165 566T175 555T183 540T186 520Q186 498 172 481T127 463Z"></path><path stroke-width="1" 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xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E193-MJMATHI-3B8" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E193-MJMAIN-31" x="663" y="-213"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E193-MJMAIN-3D" x="5405" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E193-MJMAIN-30" x="6461" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-192">\theta_0 = 360, \theta_1 =0</script> 时：</p><p><img src='image/0f38a99c8ceb8aa5b90a5f12136fdf43.png' alt='' /></p><p>大概在 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-193-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="19.142ex" height="2.461ex" viewBox="0 -806.1 8241.6 1059.4" role="img" focusable="false" style="vertical-align: -0.588ex;"><defs><path stroke-width="1" id="E194-MJMATHI-3B8" d="M35 200Q35 302 74 415T180 610T319 704Q320 704 327 704T339 705Q393 701 423 656Q462 596 462 495Q462 380 417 261T302 66T168 -10H161Q125 -10 99 10T60 63T41 130T35 200ZM383 566Q383 668 330 668Q294 668 260 623T204 521T170 421T157 371Q206 370 254 370L351 371Q352 372 359 404T375 484T383 566ZM113 132Q113 26 166 26Q181 26 198 36T239 74T287 161T335 307L340 324H145Q145 321 136 286T120 208T113 132Z"></path><path stroke-width="1" id="E194-MJMAIN-30" d="M96 585Q152 666 249 666Q297 666 345 640T423 548Q460 465 460 320Q460 165 417 83Q397 41 362 16T301 -15T250 -22Q224 -22 198 -16T137 16T82 83Q39 165 39 320Q39 494 96 585ZM321 597Q291 629 250 629Q208 629 178 597Q153 571 145 525T137 333Q137 175 145 125T181 46Q209 16 250 16Q290 16 318 46Q347 76 354 130T362 333Q362 478 354 524T321 597Z"></path><path stroke-width="1" id="E194-MJMAIN-3D" d="M56 347Q56 360 70 367H707Q722 359 722 347Q722 336 708 328L390 327H72Q56 332 56 347ZM56 153Q56 168 72 173H708Q722 163 722 153Q722 140 707 133H70Q56 140 56 153Z"></path><path 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transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E194-MJMAIN-30" x="663" y="-213"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E194-MJMAIN-3D" x="1201" y="0"></use><g transform="translate(2257,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E194-MJMAIN-30"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E194-MJMAIN-2E" x="500" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E194-MJMAIN-31" x="779" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E194-MJMAIN-32" x="1279" y="0"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E194-MJMAIN-2C" x="4037" y="0"></use><g transform="translate(4482,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E194-MJMATHI-3B8" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E194-MJMAIN-31" x="663" y="-213"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E194-MJMAIN-3D" x="5683" y="0"></use><g transform="translate(6740,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E194-MJMAIN-32"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E194-MJMAIN-35" x="500" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E194-MJMAIN-30" x="1001" y="0"></use></g></g></svg></span><script type="math/tex" id="MathJax-Element-193">\theta_0 = 0.12, \theta_1 =250</script> 时：</p><p><img src='image/20180106_092119.png' alt='' /></p><p>上图中最中心的点（红点），近乎为图像中的最低点，也即损失函数的最小值，此时对应 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-194-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="5.868ex" height="2.577ex" viewBox="0 -806.1 2526.7 1109.7" role="img" focusable="false" style="vertical-align: -0.705ex;"><defs><path stroke-width="1" id="E195-MJMATHI-68" d="M137 683Q138 683 209 688T282 694Q294 694 294 685Q294 674 258 534Q220 386 220 383Q220 381 227 388Q288 442 357 442Q411 442 444 415T478 336Q478 285 440 178T402 50Q403 36 407 31T422 26Q450 26 474 56T513 138Q516 149 519 151T535 153Q555 153 555 145Q555 144 551 130Q535 71 500 33Q466 -10 419 -10H414Q367 -10 346 17T325 74Q325 90 361 192T398 345Q398 404 354 404H349Q266 404 205 306L198 293L164 158Q132 28 127 16Q114 -11 83 -11Q69 -11 59 -2T48 16Q48 30 121 320L195 616Q195 629 188 632T149 637H128Q122 643 122 645T124 664Q129 683 137 683Z"></path><path stroke-width="1" id="E195-MJMATHI-3B8" d="M35 200Q35 302 74 415T180 610T319 704Q320 704 327 704T339 705Q393 701 423 656Q462 596 462 495Q462 380 417 261T302 66T168 -10H161Q125 -10 99 10T60 63T41 130T35 200ZM383 566Q383 668 330 668Q294 668 260 623T204 521T170 421T157 371Q206 370 254 370L351 371Q352 372 359 404T375 484T383 566ZM113 132Q113 26 166 26Q181 26 198 36T239 74T287 161T335 307L340 324H145Q145 321 136 286T120 208T113 132Z"></path><path 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289Z"></path><path stroke-width="1" id="E195-MJMAIN-29" d="M60 749L64 750Q69 750 74 750H86L114 726Q208 641 251 514T294 250Q294 182 284 119T261 12T224 -76T186 -143T145 -194T113 -227T90 -246Q87 -249 86 -250H74Q66 -250 63 -250T58 -247T55 -238Q56 -237 66 -225Q221 -64 221 250T66 725Q56 737 55 738Q55 746 60 749Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E195-MJMATHI-68" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E195-MJMATHI-3B8" x="815" y="-219"></use><g transform="translate(1175,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E195-MJMAIN-28" x="0" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E195-MJMATHI-78" x="389" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E195-MJMAIN-29" x="962" y="0"></use></g></g></svg></span><script type="math/tex" id="MathJax-Element-194">h_\theta\left(x\right)</script> 对数据的拟合情况如左图所示，嗯，一看就拟合的很不错，预测应该比较精准啦。</p><h2><a name='header-n316' class='md-header-anchor '></a>2.5 梯度下降(Gradient Descent)</h2><p>在特征量很大的情况下，即便是借用计算机来生成图像，人工的方法也很难读出 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-206-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="4.758ex" height="2.577ex" viewBox="0 -806.1 2048.7 1109.7" role="img" focusable="false" style="vertical-align: -0.705ex;"><defs><path stroke-width="1" id="E207-MJMATHI-4A" d="M447 625Q447 637 354 637H329Q323 642 323 645T325 664Q329 677 335 683H352Q393 681 498 681Q541 681 568 681T605 682T619 682Q633 682 633 672Q633 670 630 658Q626 642 623 640T604 637Q552 637 545 623Q541 610 483 376Q420 128 419 127Q397 64 333 21T195 -22Q137 -22 97 8T57 88Q57 130 80 152T132 174Q177 174 182 130Q182 98 164 80T123 56Q115 54 115 53T122 44Q148 15 197 15Q235 15 271 47T324 130Q328 142 387 380T447 625Z"></path><path stroke-width="1" id="E207-MJMAIN-28" d="M94 250Q94 319 104 381T127 488T164 576T202 643T244 695T277 729T302 750H315H319Q333 750 333 741Q333 738 316 720T275 667T226 581T184 443T167 250T184 58T225 -81T274 -167T316 -220T333 -241Q333 -250 318 -250H315H302L274 -226Q180 -141 137 -14T94 250Z"></path><path stroke-width="1" id="E207-MJMATHI-3B8" d="M35 200Q35 302 74 415T180 610T319 704Q320 704 327 704T339 705Q393 701 423 656Q462 596 462 495Q462 380 417 261T302 66T168 -10H161Q125 -10 99 10T60 63T41 130T35 200ZM383 566Q383 668 330 668Q294 668 260 623T204 521T170 421T157 371Q206 370 254 370L351 371Q352 372 359 404T375 484T383 566ZM113 132Q113 26 166 26Q181 26 198 36T239 74T287 161T335 307L340 324H145Q145 321 136 286T120 208T113 132Z"></path><path stroke-width="1" id="E207-MJMAIN-29" d="M60 749L64 750Q69 750 74 750H86L114 726Q208 641 251 514T294 250Q294 182 284 119T261 12T224 -76T186 -143T145 -194T113 -227T90 -246Q87 -249 86 -250H74Q66 -250 63 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width="1.09ex" height="2.11ex" viewBox="0 -806.1 469.5 908.7" role="img" focusable="false" style="vertical-align: -0.238ex;"><defs><path stroke-width="1" id="E66-MJMATHI-3B8" d="M35 200Q35 302 74 415T180 610T319 704Q320 704 327 704T339 705Q393 701 423 656Q462 596 462 495Q462 380 417 261T302 66T168 -10H161Q125 -10 99 10T60 63T41 130T35 200ZM383 566Q383 668 330 668Q294 668 260 623T204 521T170 421T157 371Q206 370 254 370L351 371Q352 372 359 404T375 484T383 566ZM113 132Q113 26 166 26Q181 26 198 36T239 74T287 161T335 307L340 324H145Q145 321 136 286T120 208T113 132Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E66-MJMATHI-3B8" x="0" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-65">\theta</script> 值。</strong></p><p>梯度下降背后的思想是：开始时，我们随机选择一个参数组合<span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-196-Frame" 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type="math/tex" id="MathJax-Element-196">\left( {\theta_{0}},{\theta_{1}},......,{\theta_{n}} \right)</script>即起始点，计算损失函数，然后寻找下一个能使得损失函数下降最多的参数组合。不断迭代，直到找到一个<strong>局部最小值(local minimum)</strong>，由于下降的情况只考虑当前参数组合周围的情况，所以无法确定当前的局部最小值是否就是<strong>全局最小值(global minimum)</strong>，不同的初始参数组合，可能会产生不同的局部最小值。</p><p>下图根据不同的起始点，产生了两个不同的局部最小值。</p><p><img src='image/db48c81304317847870d486ba5bb2015.jpg' alt='' /></p><p>视频中举了下山的例子，即我们在山顶上的某个位置，为了下山，就不断地看一下周围<strong>下一步往哪走</strong>下山比较快，然后就<strong>迈出那一步</strong>，一直重复，直到我们到达山下的某一处<strong>陆地</strong>。</p><p>梯度下降公式：</p><p><span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-212-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="28.85ex" height="12.149ex" viewBox="0 -2866.8 12421.4 5231" role="img" focusable="false" style="vertical-align: -5.491ex;"><defs><path stroke-width="1" id="E213-MJMAIN-72" d="M36 46H50Q89 46 97 60V68Q97 77 97 91T98 122T98 161T98 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y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E210-MJMAIN-30" x="6267" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-209">\theta_1 := \theta_1 - \alpha*0</script>）。</p><blockquote><p>不熟悉斜率的话，就当斜率的值等于图中三角形的高度除以水平长度好啦，精确地求斜率的方法是求导。</p></blockquote><p>&nbsp;</p><p>对于学习速率 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-145-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="1.488ex" height="1.41ex" viewBox="0 -504.6 640.5 607.1" role="img" focusable="false" style="vertical-align: -0.238ex;"><defs><path stroke-width="1" id="E146-MJMATHI-3B1" d="M34 156Q34 270 120 356T309 442Q379 442 421 402T478 304Q484 275 485 237V208Q534 282 560 374Q564 388 566 390T582 393Q603 393 603 385Q603 376 594 346T558 261T497 161L486 147L487 123Q489 67 495 47T514 26Q528 28 540 37T557 60Q559 67 562 68T577 70Q597 70 597 62Q597 56 591 43Q579 19 556 5T512 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/></p><p><strong>最后，梯度下降不止可以用于线性回归中的损失函数，还通用于最小化其他的损失函数。</strong></p><h2><a name='header-n385' class='md-header-anchor '></a>2.7 线性回归中的梯度下降(Gradient Descent For Linear Regression)</h2><p>线性回归模型</p><ul><li><span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-210-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="17.039ex" height="2.577ex" viewBox="0 -806.1 7336.3 1109.7" role="img" focusable="false" style="vertical-align: -0.705ex;"><defs><path stroke-width="1" id="E211-MJMATHI-68" d="M137 683Q138 683 209 688T282 694Q294 694 294 685Q294 674 258 534Q220 386 220 383Q220 381 227 388Q288 442 357 442Q411 442 444 415T478 336Q478 285 440 178T402 50Q403 36 407 31T422 26Q450 26 474 56T513 138Q516 149 519 151T535 153Q555 153 555 145Q555 144 551 130Q535 71 500 33Q466 -10 419 -10H414Q367 -10 346 17T325 74Q325 90 361 192T398 345Q398 404 354 404H349Q266 404 205 306L198 293L164 158Q132 28 127 16Q114 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id="MathJax-Element-220">\frac{\partial}{\partial\theta_1} J(\theta)=\frac{1}{m}\sum\limits_{i=1}^{m}{{\left( {{h}_{\theta }}\left( {{x}^{(i)}} \right)-{{y}^{(i)}} \right)}} *x_1^{(i)}</script></p><p>&nbsp;</p><p>上文中所提到的梯度下降，都为批量梯度下降(Batch Gradient Descent)，即每次计算都使用<strong>所有</strong>的数据集 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-221-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="6.089ex" height="5.963ex" viewBox="0 -1509.8 2621.7 2567.2" role="img" focusable="false" style="vertical-align: -2.456ex;"><defs><path stroke-width="1" id="E222-MJMAIN-28" d="M94 250Q94 319 104 381T127 488T164 576T202 643T244 695T277 729T302 750H315H319Q333 750 333 741Q333 738 316 720T275 667T226 581T184 443T167 250T184 58T225 -81T274 -167T316 -220T333 -241Q333 -250 318 -250H315H302L274 -226Q180 -141 137 -14T94 250Z"></path><path stroke-width="1" id="E222-MJSZ1-2211" d="M61 748Q64 750 489 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transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E222-MJMAIN-3D" x="345" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E222-MJMAIN-31" x="1124" y="0"></use></g><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E222-MJMATHI-6D" x="372" y="1344"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E222-MJSZ3-29" x="1885" y="-1"></use></g></svg></span><script type="math/tex" id="MathJax-Element-221">\left(\sum\limits_{i=1}^{m}\right)</script> 更新。</p><p>由于线性回归函数呈现<strong>碗状</strong>，且<strong>只有一个</strong>全局的最优值，所以函数<strong>一定总会</strong>收敛到全局最小值（学习速率不可过大）。同时，函数 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-222-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="1.471ex" height="1.994ex" viewBox="0 -755.9 633.5 858.4" role="img" focusable="false" style="vertical-align: -0.238ex;"><defs><path stroke-width="1" id="E223-MJMATHI-4A" d="M447 625Q447 637 354 637H329Q323 642 323 645T325 664Q329 677 335 683H352Q393 681 498 681Q541 681 568 681T605 682T619 682Q633 682 633 672Q633 670 630 658Q626 642 623 640T604 637Q552 637 545 623Q541 610 483 376Q420 128 419 127Q397 64 333 21T195 -22Q137 -22 97 8T57 88Q57 130 80 152T132 174Q177 174 182 130Q182 98 164 80T123 56Q115 54 115 53T122 44Q148 15 197 15Q235 15 271 47T324 130Q328 142 387 380T447 625Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E223-MJMATHI-4A" x="0" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-222">J</script> 被称为<strong>凸二次函数</strong>，而线性回归函数求解最小值问题属于<strong>凸函数优化问题</strong>。</p><p><img src='image/24e9420f16fdd758ccb7097788f879e7.png' alt='' /></p><p>另外，使用循环求解，代码较为冗余，后面会讲到如何使用<strong>向量化(Vectorization)</strong>来简化代码并优化计算，使梯度下降运行的更快更好。</p><h1><a name='header-n431' class='md-header-anchor '></a>3 Linear Algebra Review</h1><p>这部分，学过线性代数的可以复习一下，比较基础。笔记整理暂留。</p><h2><a name='header-n434' class='md-header-anchor '></a>3.1 Matrices and Vectors</h2><p>Octave/Matlab 代码:</p><pre class="md-fences md-end-block md-fences-with-lineno" lang="matlab" style="break-inside: unset;"> <div class="CodeMirror cm-s-inner CodeMirror-wrap"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 0px; left: 39px;"></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 35px; margin-bottom: 0px; border-right-width: 30px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre><div class="CodeMirror-linenumber CodeMirror-gutter-elt"><div>17</div></div></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation"><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: -35px; width: 35px;"></div><div class="CodeMirror-gutter-wrapper CodeMirror-activeline-gutter" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">1</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% The ; denotes we are going back to a new row.</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">2</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">A</span> = [<span class="cm-number">1</span>, <span class="cm-number">2</span>, <span class="cm-number">3</span>; <span class="cm-number">4</span>, <span class="cm-number">5</span>, <span class="cm-number">6</span>; <span class="cm-number">7</span>, <span class="cm-number">8</span>, <span class="cm-number">9</span>; <span class="cm-number">10</span>, <span class="cm-number">11</span>, <span class="cm-number">12</span>]</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">3</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">4</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% Initialize a vector </span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">5</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">v</span> = [<span class="cm-number">1</span>;<span class="cm-number">2</span>;<span class="cm-number">3</span>] </span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">6</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">7</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% Get the dimension of the matrix A where m = rows and n = columns</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">8</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">[<span class="cm-variable">m</span>,<span class="cm-variable">n</span>] = <span class="cm-builtin">size</span>(<span class="cm-variable">A</span>)</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">9</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">10</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% You could also store it this way</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">11</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">dim_A</span> = <span class="cm-builtin">size</span>(<span class="cm-variable">A</span>)</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">12</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">13</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% Get the dimension of the vector v </span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">14</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">dim_v</span> = <span class="cm-builtin">size</span>(<span class="cm-variable">v</span>)</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">15</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">16</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% Now let's index into the 2nd row 3rd column of matrix A</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">17</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">A_23</span> = <span class="cm-variable">A</span>(<span class="cm-number">2</span>,<span class="cm-number">3</span>)</span></pre></div></div></div></div></div></div><div style="position: absolute; height: 30px; width: 1px; border-bottom: 0px solid transparent; top: 403px;"></div><div class="CodeMirror-gutters" style="height: 433px;"><div class="CodeMirror-gutter CodeMirror-linenumbers" style="width: 34px;"></div></div></div></div></pre><p>执行结果:</p><pre class="md-fences md-end-block md-fences-with-lineno" lang="" style="break-inside: unset;"> <div class="CodeMirror cm-s-inner CodeMirror-wrap"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 0px; left: 39px;"></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 35px; margin-bottom: 0px; border-right-width: 30px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre><div class="CodeMirror-linenumber CodeMirror-gutter-elt"><div>24</div></div></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation"><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: -35px; width: 35px;"></div><div class="CodeMirror-gutter-wrapper CodeMirror-activeline-gutter" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">1</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">A =</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">2</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">3</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  1 &nbsp;  2 &nbsp;  3</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">4</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  4 &nbsp;  5 &nbsp;  6</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">5</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  7 &nbsp;  8 &nbsp;  9</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">6</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 10 &nbsp; 11 &nbsp; 12</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">7</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">8</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">v =</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">9</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">10</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 1</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">11</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 2</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">12</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 3</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">13</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">14</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">m =  4</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">15</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">n =  3</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">16</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">dim_A =</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">17</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">18</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 4 &nbsp; 3</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">19</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">20</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">dim_v =</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">21</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">22</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 3 &nbsp; 1</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">23</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">24</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">A_23 =  6</span></pre></div></div></div></div></div></div><div style="position: absolute; height: 30px; width: 1px; border-bottom: 0px solid transparent; top: 538px;"></div><div class="CodeMirror-gutters" style="height: 568px;"><div class="CodeMirror-gutter CodeMirror-linenumbers" style="width: 34px;"></div></div></div></div></pre><h2><a name='header-n441' class='md-header-anchor '></a>3.2 Addition and Scalar Multiplication</h2><p>Octave/Matlab 代码:</p><pre class="md-fences md-end-block md-fences-with-lineno" lang="matlab" style="break-inside: unset;"> <div class="CodeMirror cm-s-inner CodeMirror-wrap"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 0px; left: 39px;"></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 35px; margin-bottom: 0px; border-right-width: 30px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre><div class="CodeMirror-linenumber CodeMirror-gutter-elt"><div>21</div></div></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation"><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: -35px; width: 35px;"></div><div class="CodeMirror-gutter-wrapper CodeMirror-activeline-gutter" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">1</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% Initialize matrix A and B </span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">2</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">A</span> = [<span class="cm-number">1</span>, <span class="cm-number">2</span>, <span class="cm-number">4</span>; <span class="cm-number">5</span>, <span class="cm-number">3</span>, <span class="cm-number">2</span>]</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">3</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">B</span> = [<span class="cm-number">1</span>, <span class="cm-number">3</span>, <span class="cm-number">4</span>; <span class="cm-number">1</span>, <span class="cm-number">1</span>, <span class="cm-number">1</span>]</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">4</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">5</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% Initialize constant s </span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">6</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">s</span> = <span class="cm-number">2</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">7</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">8</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% See how element-wise addition works</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">9</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">add_AB</span> = <span class="cm-variable">A</span> <span class="cm-operator">+</span> <span class="cm-variable">B</span> </span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">10</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">11</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% See how element-wise subtraction works</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">12</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">sub_AB</span> = <span class="cm-variable">A</span> <span class="cm-operator">-</span> <span class="cm-variable">B</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">13</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">14</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% See how scalar multiplication works</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">15</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">mult_As</span> = <span class="cm-variable">A</span> <span class="cm-operator">*</span> <span class="cm-variable">s</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">16</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">17</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% Divide A by s</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">18</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">div_As</span> = <span class="cm-variable">A</span> <span class="cm-operator">/</span> <span class="cm-variable">s</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">19</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">20</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% What happens if we have a Matrix + scalar?</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">21</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">add_As</span> = <span class="cm-variable">A</span> <span class="cm-operator">+</span> <span class="cm-variable">s</span></span></pre></div></div></div></div></div></div><div style="position: absolute; height: 30px; width: 1px; border-bottom: 0px solid transparent; top: 470px;"></div><div class="CodeMirror-gutters" style="height: 500px;"><div class="CodeMirror-gutter CodeMirror-linenumbers" style="width: 34px;"></div></div></div></div></pre><p>执行结果:</p><pre class="md-fences md-end-block md-fences-with-lineno" lang="" style="break-inside: unset;"> <div class="CodeMirror cm-s-inner CodeMirror-wrap"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 0px; left: 39px;"></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 35px; margin-bottom: 0px; border-right-width: 30px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre><div class="CodeMirror-linenumber CodeMirror-gutter-elt"><div>35</div></div></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation"><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: -35px; width: 35px;"></div><div class="CodeMirror-gutter-wrapper CodeMirror-activeline-gutter" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">1</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">A =</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">2</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">3</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 1 &nbsp; 2 &nbsp; 4</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">4</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 5 &nbsp; 3 &nbsp; 2</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">5</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">6</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">B =</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">7</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">8</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 1 &nbsp; 3 &nbsp; 4</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">9</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 1 &nbsp; 1 &nbsp; 1</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">10</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">11</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">s =  2</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">12</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">add_AB =</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">13</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">14</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 2 &nbsp; 5 &nbsp; 8</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">15</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 6 &nbsp; 4 &nbsp; 3</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">16</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">17</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">sub_AB =</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">18</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">19</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 0  -1 &nbsp; 0</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">20</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 4 &nbsp; 2 &nbsp; 1</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">21</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">22</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">mult_As =</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">23</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">24</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  2 &nbsp;  4 &nbsp;  8</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">25</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 10 &nbsp;  6 &nbsp;  4</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">26</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">27</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">div_As =</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">28</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">29</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 0.50000 &nbsp; 1.00000 &nbsp; 2.00000</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">30</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 2.50000 &nbsp; 1.50000 &nbsp; 1.00000</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">31</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">32</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">add_As =</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">33</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">34</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 3 &nbsp; 4 &nbsp; 6</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">35</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 7 &nbsp; 5 &nbsp; 4</span></pre></div></div></div></div></div></div><div style="position: absolute; height: 30px; width: 1px; border-bottom: 0px solid transparent; top: 784px;"></div><div class="CodeMirror-gutters" style="height: 814px;"><div class="CodeMirror-gutter CodeMirror-linenumbers" style="width: 34px;"></div></div></div></div></pre><h2><a name='header-n448' class='md-header-anchor '></a>3.3 Matrix Vector Multiplication</h2><p>Octave/Matlab 代码:</p><pre class="md-fences md-end-block md-fences-with-lineno" lang="matlab"> <div class="CodeMirror cm-s-inner CodeMirror-wrap"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 0px; left: 39px;"></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 35px; margin-bottom: 0px; border-right-width: 30px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre><div class="CodeMirror-linenumber CodeMirror-gutter-elt"><div>10</div></div></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation"><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: -35px; width: 35px;"></div><div class="CodeMirror-gutter-wrapper CodeMirror-activeline-gutter" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">1</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% Initialize matrix A </span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">2</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">A</span> = [<span class="cm-number">1</span>, <span class="cm-number">2</span>, <span class="cm-number">3</span>; <span class="cm-number">4</span>, <span class="cm-number">5</span>, <span class="cm-number">6</span>;<span class="cm-number">7</span>, <span class="cm-number">8</span>, <span class="cm-number">9</span>] </span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">3</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">4</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% Initialize vector v </span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">5</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">v</span> = [<span class="cm-number">1</span>; <span class="cm-number">1</span>; <span class="cm-number">1</span>] </span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">6</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">7</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% Multiply A * v</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">8</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">Av</span> = <span class="cm-variable">A</span> <span class="cm-operator">*</span> <span class="cm-variable">v</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">9</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">10</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div></div></div></div></div></div><div style="position: absolute; height: 30px; width: 1px; border-bottom: 0px solid transparent; top: 224px;"></div><div class="CodeMirror-gutters" style="height: 254px;"><div class="CodeMirror-gutter CodeMirror-linenumbers" style="width: 34px;"></div></div></div></div></pre><p>执行结果:</p><pre class="md-fences md-end-block md-fences-with-lineno" lang="" style="break-inside: unset;"> <div class="CodeMirror cm-s-inner CodeMirror-wrap"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 0px; left: 39px;"></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 35px; margin-bottom: 0px; border-right-width: 30px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre><div class="CodeMirror-linenumber CodeMirror-gutter-elt"><div>18</div></div></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation"><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: -35px; width: 35px;"></div><div class="CodeMirror-gutter-wrapper CodeMirror-activeline-gutter" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">1</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">A =</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">2</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">3</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 1 &nbsp; 2 &nbsp; 3</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">4</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 4 &nbsp; 5 &nbsp; 6</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">5</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 7 &nbsp; 8 &nbsp; 9</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">6</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">7</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">v =</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">8</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">9</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 1</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">10</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 1</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">11</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 1</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">12</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">13</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">Av =</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">14</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">15</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  6</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">16</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 15</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">17</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 24</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">18</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div></div></div></div></div></div><div style="position: absolute; height: 30px; width: 1px; border-bottom: 0px solid transparent; top: 403px;"></div><div class="CodeMirror-gutters" style="height: 433px;"><div class="CodeMirror-gutter CodeMirror-linenumbers" style="width: 34px;"></div></div></div></div></pre><h2><a name='header-n455' class='md-header-anchor '></a>3.4 Matrix Matrix Multiplication</h2><p>Octave/Matlab 代码:</p><pre class="md-fences md-end-block md-fences-with-lineno" lang="matlab"> <div class="CodeMirror cm-s-inner CodeMirror-wrap"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 0px; left: 39px;"></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 35px; margin-bottom: 0px; border-right-width: 30px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre><div class="CodeMirror-linenumber CodeMirror-gutter-elt"><div>10</div></div></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation"><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: -35px; width: 35px;"></div><div class="CodeMirror-gutter-wrapper CodeMirror-activeline-gutter" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">1</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% Initialize a 3 by 2 matrix </span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">2</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">A</span> = [<span class="cm-number">1</span>, <span class="cm-number">2</span>; <span class="cm-number">3</span>, <span class="cm-number">4</span>;<span class="cm-number">5</span>, <span class="cm-number">6</span>]</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">3</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">4</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% Initialize a 2 by 1 matrix </span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">5</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">B</span> = [<span class="cm-number">1</span>; <span class="cm-number">2</span>] </span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">6</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">7</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% We expect a resulting matrix of (3 by 2)*(2 by 1) = (3 by 1) </span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">8</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">mult_AB</span> = <span class="cm-variable">A</span><span class="cm-operator">*</span><span class="cm-variable">B</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">9</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">10</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% Make sure you understand why we got that result</span></span></pre></div></div></div></div></div></div><div style="position: absolute; height: 30px; width: 1px; border-bottom: 0px solid transparent; top: 224px;"></div><div class="CodeMirror-gutters" style="height: 254px;"><div class="CodeMirror-gutter CodeMirror-linenumbers" style="width: 34px;"></div></div></div></div></pre><p>执行结果:</p><pre class="md-fences md-end-block md-fences-with-lineno" lang="" style="break-inside: unset;"> <div class="CodeMirror cm-s-inner CodeMirror-wrap"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 0px; left: 39px;"></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 35px; margin-bottom: 0px; border-right-width: 30px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre><div class="CodeMirror-linenumber CodeMirror-gutter-elt"><div>17</div></div></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation"><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: -35px; width: 35px;"></div><div class="CodeMirror-gutter-wrapper CodeMirror-activeline-gutter" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">1</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">A =</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">2</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">3</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 1 &nbsp; 2</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">4</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 3 &nbsp; 4</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">5</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 5 &nbsp; 6</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">6</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">7</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">B =</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">8</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">9</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 1</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">10</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 2</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">11</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">12</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">mult_AB =</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">13</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">14</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  5</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">15</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 11</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">16</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 17</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">17</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div></div></div></div></div></div><div style="position: absolute; height: 30px; width: 1px; border-bottom: 0px solid transparent; top: 381px;"></div><div class="CodeMirror-gutters" style="height: 411px;"><div class="CodeMirror-gutter CodeMirror-linenumbers" style="width: 34px;"></div></div></div></div></pre><h2><a name='header-n462' class='md-header-anchor '></a>3.5 Matrix Multiplication Properties</h2><p>Octave/Matlab 代码:</p><pre class="md-fences md-end-block md-fences-with-lineno" lang="matlab" style="break-inside: unset;"> <div class="CodeMirror cm-s-inner CodeMirror-wrap"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 0px; left: 39px;"></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 35px; margin-bottom: 0px; border-right-width: 30px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre><div class="CodeMirror-linenumber CodeMirror-gutter-elt"><div>22</div></div></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation"><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: -35px; width: 35px;"></div><div class="CodeMirror-gutter-wrapper CodeMirror-activeline-gutter" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">1</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% Initialize random matrices A and B </span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">2</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">A</span> = [<span class="cm-number">1</span>,<span class="cm-number">2</span>;<span class="cm-number">4</span>,<span class="cm-number">5</span>]</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">3</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">B</span> = [<span class="cm-number">1</span>,<span class="cm-number">1</span>;<span class="cm-number">0</span>,<span class="cm-number">2</span>]</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">4</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">5</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% Initialize a 2 by 2 identity matrix</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">6</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">I</span> = <span class="cm-builtin">eye</span>(<span class="cm-number">2</span>)</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">7</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">8</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% The above notation is the same as I = [1,0;0,1]</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">9</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">10</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% What happens when we multiply I*A ? </span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">11</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">IA</span> = <span class="cm-variable">I</span><span class="cm-operator">*</span><span class="cm-variable">A</span> </span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">12</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">13</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% How about A*I ? </span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">14</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">AI</span> = <span class="cm-variable">A</span><span class="cm-operator">*</span><span class="cm-variable">I</span> </span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">15</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">16</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% Compute A*B </span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">17</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">AB</span> = <span class="cm-variable">A</span><span class="cm-operator">*</span><span class="cm-variable">B</span> </span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">18</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">19</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% Is it equal to B*A? </span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">20</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">BA</span> = <span class="cm-variable">B</span><span class="cm-operator">*</span><span class="cm-variable">A</span> </span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">21</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">22</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% Note that IA = AI but AB != BA</span></span></pre></div></div></div></div></div></div><div style="position: absolute; height: 30px; width: 1px; border-bottom: 0px solid transparent; top: 493px;"></div><div class="CodeMirror-gutters" style="height: 523px;"><div class="CodeMirror-gutter CodeMirror-linenumbers" style="width: 34px;"></div></div></div></div></pre><p>执行结果:</p><pre class="md-fences md-end-block md-fences-with-lineno" lang="" style="break-inside: unset;"> <div class="CodeMirror cm-s-inner CodeMirror-wrap"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 0px; left: 39px;"></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 35px; margin-bottom: 0px; border-right-width: 30px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre><div class="CodeMirror-linenumber CodeMirror-gutter-elt"><div>36</div></div></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation"><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: -35px; width: 35px;"></div><div class="CodeMirror-gutter-wrapper CodeMirror-activeline-gutter" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">1</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">A =</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">2</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">3</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 1 &nbsp; 2</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">4</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 4 &nbsp; 5</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">5</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">6</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">B =</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">7</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">8</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 1 &nbsp; 1</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">9</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 0 &nbsp; 2</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">10</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">11</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">I =</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">12</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">13</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">Diagonal Matrix</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">14</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">15</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 1 &nbsp; 0</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">16</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 0 &nbsp; 1</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">17</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">18</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">IA =</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">19</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">20</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 1 &nbsp; 2</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">21</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 4 &nbsp; 5</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">22</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">23</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">AI =</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">24</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">25</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 1 &nbsp; 2</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">26</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 4 &nbsp; 5</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">27</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">28</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">AB =</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">29</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">30</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  1 &nbsp;  5</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">31</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  4 &nbsp; 14</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">32</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">33</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">BA =</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">34</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">35</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  5 &nbsp;  7</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">36</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  8 &nbsp; 10</span></pre></div></div></div></div></div></div><div style="position: absolute; height: 30px; width: 1px; border-bottom: 0px solid transparent; top: 806px;"></div><div class="CodeMirror-gutters" style="height: 836px;"><div class="CodeMirror-gutter CodeMirror-linenumbers" style="width: 34px;"></div></div></div></div></pre><h2><a name='header-n469' class='md-header-anchor '></a>3.6 Inverse and Transpose</h2><p>Octave/Matlab 代码:</p><pre class="md-fences md-end-block md-fences-with-lineno" lang="matlab"> <div class="CodeMirror cm-s-inner CodeMirror-wrap"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 0px; left: 39px;"></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 35px; margin-bottom: 0px; border-right-width: 30px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre><div class="CodeMirror-linenumber CodeMirror-gutter-elt"><div>13</div></div></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation"><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: -35px; width: 35px;"></div><div class="CodeMirror-gutter-wrapper CodeMirror-activeline-gutter" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">1</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% Initialize matrix A </span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">2</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">A</span> = [<span class="cm-number">1</span>,<span class="cm-number">2</span>,<span class="cm-number">0</span>;<span class="cm-number">0</span>,<span class="cm-number">5</span>,<span class="cm-number">6</span>;<span class="cm-number">7</span>,<span class="cm-number">0</span>,<span class="cm-number">9</span>]</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">3</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">4</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% Transpose A </span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">5</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">A_trans</span> = <span class="cm-variable">A</span><span class="cm-operator">'</span> </span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">6</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">7</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% Take the inverse of A </span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">8</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">A_inv</span> = <span class="cm-builtin">inv</span>(<span class="cm-variable">A</span>)</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">9</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">10</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">% What is A^(-1)*A? </span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">11</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">A_invA</span> = <span class="cm-builtin">inv</span>(<span class="cm-variable">A</span>)<span class="cm-operator">*</span><span class="cm-variable">A</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">12</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">13</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div></div></div></div></div></div><div style="position: absolute; height: 30px; width: 1px; border-bottom: 0px solid transparent; top: 291px;"></div><div class="CodeMirror-gutters" style="height: 321px;"><div class="CodeMirror-gutter CodeMirror-linenumbers" style="width: 34px;"></div></div></div></div></pre><p>执行结果:</p><pre class="md-fences md-end-block md-fences-with-lineno" lang="" style="break-inside: unset;"> <div class="CodeMirror cm-s-inner CodeMirror-wrap"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 0px; left: 39px;"></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 35px; margin-bottom: 0px; border-right-width: 30px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre><div class="CodeMirror-linenumber CodeMirror-gutter-elt"><div>24</div></div></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation"><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: -35px; width: 35px;"></div><div class="CodeMirror-gutter-wrapper CodeMirror-activeline-gutter" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">1</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">A =</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">2</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">3</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 1 &nbsp; 2 &nbsp; 0</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">4</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 0 &nbsp; 5 &nbsp; 6</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">5</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 7 &nbsp; 0 &nbsp; 9</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">6</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">7</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">A_trans =</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">8</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">9</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 1 &nbsp; 0 &nbsp; 7</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">10</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 2 &nbsp; 5 &nbsp; 0</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">11</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 0 &nbsp; 6 &nbsp; 9</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">12</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">13</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">A_inv =</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">14</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">15</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 0.348837  -0.139535 &nbsp; 0.093023</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">16</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 0.325581 &nbsp; 0.069767  -0.046512</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">17</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">  -0.271318 &nbsp; 0.108527 &nbsp; 0.038760</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">18</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">19</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">A_invA =</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">20</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">21</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 1.00000  -0.00000 &nbsp; 0.00000</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">22</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; 0.00000 &nbsp; 1.00000  -0.00000</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt" style="left: 0px; width: 26px;">23</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">  -0.00000 &nbsp; 0.00000 &nbsp; 1.00000</span></pre></div><div style="position: relative;"><div class="CodeMirror-gutter-wrapper" style="left: -35px;"><div class="CodeMirror-linenumber CodeMirror-gutter-elt CodeMirror-linenumber-show" style="left: 0px; width: 26px;">24</div></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div></div></div></div></div></div><div style="position: absolute; height: 30px; width: 1px; border-bottom: 0px solid transparent; top: 538px;"></div><div class="CodeMirror-gutters" style="height: 568px;"><div class="CodeMirror-gutter CodeMirror-linenumbers" style="width: 34px;"></div></div></div></div></pre></div>
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diff --git a/week1.md b/week1.md
index 3c25be8..0bed38f 100644
--- a/week1.md
+++ b/week1.md
@@ -144,9 +144,9 @@
 
 ![](image/20180105_212048.png)
 
-其中 $h$ 代表结果函数，也称为**假设(hypothesis)** 。这个函数 $h$ 根据输入(房屋的面积)，给出预测结果输出(房屋的价格)，即是一个 $X\to Y$ 的映射。
+其中 $h$ 代表结果函数，也称为**假设(hypothesis)** 。假设函数根据输入(房屋的面积)，给出预测结果输出(房屋的价格)，即是一个 $X\to Y$ 的映射。
 
-$h_\theta(x)=\theta_0+\theta_1x$，为其中一种可行的表达式。
+$h_\theta(x)=\theta_0+\theta_1x$，为解决房价问题的一种可行表达式。
 
 > $x$: 特征/输入变量。
 
@@ -156,7 +156,7 @@ $h_\theta(x)=\theta_0+\theta_1x$，为其中一种可行的表达式。
 
 ## 2.2 损失函数(Cost Function)
 
-我们的目的在于求解预测结果 $h_\theta(x)$  最接近于实际结果 $y$ 时 $\theta$ 的取值，则问题可表达为**求解 $\sum\limits_{i=0}^{m}(h_\theta(x^{(i)})-y^{(i)})$ 的最小值**。
+我们的目的在于求解预测结果 $h$ 最接近于实际结果 $y$ 时 $\theta$ 的取值，则问题可表达为**求解 $\sum\limits_{i=0}^{m}(h_\theta(x^{(i)})-y^{(i)})$ 的最小值**。
 
 > $m$: 训练集中的样本总数
 >
@@ -174,12 +174,12 @@ $h_\theta(x)=\theta_0+\theta_1x$，为其中一种可行的表达式。
 
 $$J(\theta_0,\theta_1)=\dfrac{1}{2m}\displaystyle\sum_{i=1}^m\left(\hat{y}_{i}-y_{i} \right)^2=\dfrac{1}{2m}\displaystyle\sum_{i=1}^m\left(h_\theta(x_{i})-y_{i}\right)^2$$ 
 
+> $\hat{y}$: $y$ 的预测值
+>
 > 系数 $\frac{1}{2}$ 存在与否都不会影响结果，这里是为了在应用梯度下降时便于求解，平方的导数会抵消掉 $\frac{1}{2}$ 。
 
 讨论到这里，我们的问题就转化成了**求解 $J\left( \theta_0, \theta_1  \right)$ 的最小值**。
 
-
-
 ## 2.3 损失函数 - 直观理解1(Cost Function - Intuition I)
 
 根据上节视频，列出如下定义：
@@ -299,7 +299,7 @@ $\begin{align*} & \text{repeat until convergence:} \; \lbrace \newline \; &{{\th
 
 ![](image/20180106_203726.png)
 
-对于 $j = 0, j = 1$ 时，给出偏导计算公式的推导过程如下：
+当 $j = 0, j = 1$ 时，**平方损失函数求导的推导过程：**
 
 $\frac{\partial}{\partial\theta_j} J(\theta_1, \theta_2)=\frac{\partial}{\partial\theta_j} \left(\frac{1}{2m}\sum\limits_{i=1}^{m}{{\left( {{h}_{\theta }}\left( {{x}^{(i)}} \right)-{{y}^{(i)}} \right)}^{2}} \right)=$
 
@@ -319,7 +319,7 @@ $\frac{\partial}{\partial\theta_1} J(\theta)=\frac{1}{m}\sum\limits_{i=1}^{m}{{\
 
 上文中所提到的梯度下降，都为批量梯度下降(Batch Gradient Descent)，即每次计算都使用**所有**的数据集 $\left(\sum\limits_{i=1}^{m}\right)$ 更新。
 
-由于线性回归函数呈现**碗状**，且**只有一个**全局的最优值，所以函数**一定总会**收敛到全局最小值（学习速率不可过大）。同时，函数 $J​$ 被称为凸二次函数，而线性回归函数求解最小值问题属于**凸函数优化问题**。
+由于线性回归函数呈现**碗状**，且**只有一个**全局的最优值，所以函数**一定总会**收敛到全局最小值（学习速率不可过大）。同时，函数 $J$ 被称为**凸二次函数**，而线性回归函数求解最小值问题属于**凸函数优化问题**。
 
 ![](image/24e9420f16fdd758ccb7097788f879e7.png)
 
diff --git a/week2.html b/week2.html
index d23b07f..31f91ff 100644
--- a/week2.html
+++ b/week2.html
@@ -220,8 +220,8 @@ header, .context-menu, .megamenu-content, footer { font-family: "Segoe UI", Aria
 </style>
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 <body class='typora-export' >
-<div  id='write'  class = 'is-node show-fences-line-number'><div class='md-toc' mdtype='toc'><p class="md-toc-content"><span class="md-toc-item md-toc-h1" data-ref="n2"><a class="md-toc-inner" href="#header-n2">4 多变量线性回归(Linear Regression with Multiple Variables)</a></span><span class="md-toc-item md-toc-h2" data-ref="n3"><a class="md-toc-inner" href="#header-n3">4.1 多特征(Multiple Features)</a></span><span class="md-toc-item md-toc-h2" data-ref="n32"><a class="md-toc-inner" href="#header-n32">4.2 多变量梯度下降(Gradient Descent for Multiple Variables)</a></span><span class="md-toc-item md-toc-h2" data-ref="n54"><a class="md-toc-inner" href="#header-n54">4.3 梯度下降实践1-特征值缩放(Gradient Descent in Practice I - Feature Scaling)</a></span><span class="md-toc-item md-toc-h2" data-ref="n75"><a class="md-toc-inner" href="#header-n75">4.4 梯度下降实践2-学习速率(Gradient Descent in Practice II - Learning Rate)</a></span><span class="md-toc-item md-toc-h2" data-ref="n114"><a class="md-toc-inner" href="#header-n114">4.5 特征和多项式回归(Features and Polynomial Regression)</a></span><span class="md-toc-item md-toc-h2" data-ref="n129"><a class="md-toc-inner" href="#header-n129">4.6 正规方程(Normal Equation)</a></span><span class="md-toc-item md-toc-h2" data-ref="n194"><a class="md-toc-inner" href="#header-n194">4.7 不可逆性正规方程(Normal Equation Noninvertibility)</a></span><span class="md-toc-item md-toc-h1" data-ref="n226"><a class="md-toc-inner" href="#header-n226">5 Octave/Matlab Tutorial</a></span><span class="md-toc-item md-toc-h2" data-ref="n229"><a class="md-toc-inner" href="#header-n229">5.1 Basic Operations</a></span><span class="md-toc-item md-toc-h2" data-ref="n230"><a class="md-toc-inner" href="#header-n230">5.2 Moving Data Around</a></span><span class="md-toc-item md-toc-h2" data-ref="n231"><a class="md-toc-inner" href="#header-n231">5.3 Computing on Data</a></span><span class="md-toc-item md-toc-h2" data-ref="n232"><a class="md-toc-inner" href="#header-n232">5.4 Plotting Data</a></span><span class="md-toc-item md-toc-h2" data-ref="n233"><a class="md-toc-inner" href="#header-n233">5.5 Control Statements_ for, while, if statement</a></span><span class="md-toc-item md-toc-h2" data-ref="n234"><a class="md-toc-inner" href="#header-n234">5.6 Vectorization</a></span><span class="md-toc-item md-toc-h2" data-ref="n235"><a class="md-toc-inner" href="#header-n235">5.x 常用函数整理</a></span></p></div><h1><a name='header-n2' class='md-header-anchor '></a>4 多变量线性回归(Linear Regression with Multiple Variables)</h1><h2><a name='header-n3' class='md-header-anchor '></a>4.1 多特征(Multiple Features)</h2><p>对于一个要度量的对象，一般来说会有不同维度的多个特征。比如之前的房屋价格预测例子中，除了房屋的面积大小，可能还有房屋的年限、房屋的层数等等其他特征：</p><p><img src='image/20180107_234509.png' alt='' /></p><p>这里由于特征不再只有一个，引入一些新的记号</p><blockquote><p><span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-146-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="1.395ex" height="1.438ex" viewBox="0 -505.2 600.5 619.1" role="img" focusable="false" style="vertical-align: -0.264ex;"><defs><path stroke-width="1" id="E147-MJMATHI-6E" d="M21 287Q22 293 24 303T36 341T56 388T89 425T135 442Q171 442 195 424T225 390T231 369Q231 367 232 367L243 378Q304 442 382 442Q436 442 469 415T503 336T465 179T427 52Q427 26 444 26Q450 26 453 27Q482 32 505 65T540 145Q542 153 560 153Q580 153 580 145Q580 144 576 130Q568 101 554 73T508 17T439 -10Q392 -10 371 17T350 73Q350 92 386 193T423 345Q423 404 379 404H374Q288 404 229 303L222 291L189 157Q156 26 151 16Q138 -11 108 -11Q95 -11 87 -5T76 7T74 17Q74 30 112 180T152 343Q153 348 153 366Q153 405 129 405Q91 405 66 305Q60 285 60 284Q58 278 41 278H27Q21 284 21 287Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E147-MJMATHI-6E" x="0" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-146">n</script>: 特征的总数 </p><p> <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-109-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="3.409ex" height="2.461ex" viewBox="0 -956.9 1467.6 1059.4" role="img" focusable="false" style="vertical-align: -0.238ex;"><defs><path stroke-width="1" id="E110-MJMATHI-78" d="M52 289Q59 331 106 386T222 442Q257 442 286 424T329 379Q371 442 430 442Q467 442 494 420T522 361Q522 332 508 314T481 292T458 288Q439 288 427 299T415 328Q415 374 465 391Q454 404 425 404Q412 404 406 402Q368 386 350 336Q290 115 290 78Q290 50 306 38T341 26Q378 26 414 59T463 140Q466 150 469 151T485 153H489Q504 153 504 145Q504 144 502 134Q486 77 440 33T333 -11Q263 -11 227 52Q186 -10 133 -10H127Q78 -10 57 16T35 71Q35 103 54 123T99 143Q142 143 142 101Q142 81 130 66T107 46T94 41L91 40Q91 39 97 36T113 29T132 26Q168 26 194 71Q203 87 217 139T245 247T261 313Q266 340 266 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transform="translate(0,-3446.188741721854) scale(1,2.8112582781456954)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E112-MJSZ4-23A5"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E112-MJSZ4-23A6" x="0" y="-4555"></use></g></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E112-MJMAIN-2C" x="6345" y="0"></use><g transform="translate(6790,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E112-MJMATHI-78" x="0" y="0"></use><g transform="translate(572,521)"><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E112-MJMAIN-28" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E112-MJMAIN-32" x="389" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E112-MJMAIN-29" x="890" y="0"></use></g><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E112-MJMAIN-31" x="809" y="-435"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E112-MJMAIN-3D" x="8645" y="0"></use><g transform="translate(9701,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E112-MJMAIN-31"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E112-MJMAIN-34" x="500" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E112-MJMAIN-31" x="1001" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E112-MJMAIN-36" x="1501" y="0"></use></g></g></svg></span><script type="math/tex" id="MathJax-Element-111">{x}^{(2)}\text{=}\begin{bmatrix} 1416\\\ 3\\\ 2\\\ 40 \end{bmatrix}, {x}^{(2)}_{1} = 1416</script></p><p>多变量假设函数 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-42-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="1.339ex" height="1.994ex" viewBox="0 -755.9 576.5 858.4" role="img" focusable="false" style="vertical-align: -0.238ex;"><defs><path stroke-width="1" id="E42-MJMATHI-68" d="M137 683Q138 683 209 688T282 694Q294 694 294 685Q294 674 258 534Q220 386 220 383Q220 381 227 388Q288 442 357 442Q411 442 444 415T478 336Q478 285 440 178T402 50Q403 36 407 31T422 26Q450 26 474 56T513 138Q516 149 519 151T535 153Q555 153 555 145Q555 144 551 130Q535 71 500 33Q466 -10 419 -10H414Q367 -10 346 17T325 74Q325 90 361 192T398 345Q398 404 354 404H349Q266 404 205 306L198 293L164 158Q132 28 127 16Q114 -11 83 -11Q69 -11 59 -2T48 16Q48 30 121 320L195 616Q195 629 188 632T149 637H128Q122 643 122 645T124 664Q129 683 137 683Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E42-MJMATHI-68" x="0" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-42">h</script> 表示为：<span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-112-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="37.425ex" height="2.577ex" viewBox="0 -806.1 16113.4 1109.7" role="img" focusable="false" style="vertical-align: -0.705ex;"><defs><path stroke-width="1" id="E113-MJMATHI-68" d="M137 683Q138 683 209 688T282 694Q294 694 294 685Q294 674 258 534Q220 386 220 383Q220 381 227 388Q288 442 357 442Q411 442 444 415T478 336Q478 285 440 178T402 50Q403 36 407 31T422 26Q450 26 474 56T513 138Q516 149 519 151T535 153Q555 153 555 145Q555 144 551 130Q535 71 500 33Q466 -10 419 -10H414Q367 -10 346 17T325 74Q325 90 361 192T398 345Q398 404 354 404H349Q266 404 205 306L198 293L164 158Q132 28 127 16Q114 -11 83 -11Q69 -11 59 -2T48 16Q48 30 121 320L195 616Q195 629 188 632T149 637H128Q122 643 122 645T124 664Q129 683 137 683Z"></path><path stroke-width="1" id="E113-MJMATHI-3B8" d="M35 200Q35 302 74 415T180 610T319 704Q320 704 327 704T339 705Q393 701 423 656Q462 596 462 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x="663" y="-213"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E113-MJMAIN-2B" x="5006" y="0"></use><g transform="translate(6007,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E113-MJMATHI-3B8" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E113-MJMAIN-31" x="663" y="-213"></use></g><g transform="translate(6930,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E113-MJMATHI-78" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E113-MJMAIN-31" x="809" y="-213"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E113-MJMAIN-2B" x="8179" y="0"></use><g transform="translate(9179,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E113-MJMATHI-3B8" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E113-MJMAIN-32" x="663" y="-213"></use></g><g transform="translate(10103,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E113-MJMATHI-78" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E113-MJMAIN-32" x="809" y="-213"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E113-MJMAIN-2B" x="11129" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E113-MJMAIN-2E" x="11908" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E113-MJMAIN-2E" x="12353" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E113-MJMAIN-2E" x="12798" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E113-MJMAIN-2B" x="13243" y="0"></use><g transform="translate(14022,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E113-MJMATHI-3B8" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E113-MJMATHI-6E" x="663" y="-213"></use></g><g transform="translate(15016,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E113-MJMATHI-78" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E113-MJMATHI-6E" x="809" y="-213"></use></g></g></svg></span><script type="math/tex" id="MathJax-Element-112">h_{\theta}\left( x \right)={\theta_{0}}+{\theta_{1}}{x_{1}}+{\theta_{2}}{x_{2}}+...+{\theta_{n}}{x_{n}}</script></p><p>对于 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-43-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="2.145ex" height="2.461ex" viewBox="0 -806.1 923.4 1059.4" role="img" focusable="false" style="vertical-align: -0.588ex;"><defs><path stroke-width="1" id="E43-MJMATHI-3B8" d="M35 200Q35 302 74 415T180 610T319 704Q320 704 327 704T339 705Q393 701 423 656Q462 596 462 495Q462 380 417 261T302 66T168 -10H161Q125 -10 99 10T60 63T41 130T35 200ZM383 566Q383 668 330 668Q294 668 260 623T204 521T170 421T157 371Q206 370 254 370L351 371Q352 372 359 404T375 484T383 566ZM113 132Q113 26 166 26Q181 26 198 36T239 74T287 161T335 307L340 324H145Q145 321 136 286T120 208T113 132Z"></path><path stroke-width="1" id="E43-MJMAIN-30" d="M96 585Q152 666 249 666Q297 666 345 640T423 548Q460 465 460 320Q460 165 417 83Q397 41 362 16T301 -15T250 -22Q224 -22 198 -16T137 16T82 83Q39 165 39 320Q39 494 96 585ZM321 597Q291 629 250 629Q208 629 178 597Q153 571 145 525T137 333Q137 175 145 125T181 46Q209 16 250 16Q290 16 318 46Q347 76 354 130T362 333Q362 478 354 524T321 597Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E43-MJMATHI-3B8" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E43-MJMAIN-30" x="663" y="-213"></use></g></svg></span><script type="math/tex" id="MathJax-Element-43">\theta_0</script>，和单特征中一样，我们将其看作基础数值。例如，房价的基础价格。</p><p>参数向量的维度为 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-115-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="5.398ex" height="2.11ex" viewBox="0 -755.9 2323.9 908.7" role="img" focusable="false" style="vertical-align: -0.355ex;"><defs><path stroke-width="1" id="E116-MJMATHI-6E" d="M21 287Q22 293 24 303T36 341T56 388T89 425T135 442Q171 442 195 424T225 390T231 369Q231 367 232 367L243 378Q304 442 382 442Q436 442 469 415T503 336T465 179T427 52Q427 26 444 26Q450 26 453 27Q482 32 505 65T540 145Q542 153 560 153Q580 153 580 145Q580 144 576 130Q568 101 554 73T508 17T439 -10Q392 -10 371 17T350 73Q350 92 386 193T423 345Q423 404 379 404H374Q288 404 229 303L222 291L189 157Q156 26 151 16Q138 -11 108 -11Q95 -11 87 -5T76 7T74 17Q74 30 112 180T152 343Q153 348 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xlink:href="#E128-MJMAIN-31" x="3307" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E128-MJMAIN-2C" x="3808" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E128-MJMAIN-31" x="4253" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E128-MJMAIN-29" x="4754" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-127">x_i \in (-1,1)</script></p><p>对于特征的范围，并不一定需要使得 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-128-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="11.66ex" height="2.227ex" viewBox="0 -755.9 5020.1 958.9" role="img" focusable="false" style="vertical-align: -0.472ex;"><defs><path stroke-width="1" id="E129-MJMAIN-2212" d="M84 237T84 250T98 270H679Q694 262 694 250T679 230H98Q84 237 84 250Z"></path><path stroke-width="1" id="E129-MJMAIN-31" d="M213 578L200 573Q186 568 160 563T102 556H83V602H102Q149 604 189 617T245 641T273 663Q275 666 285 666Q294 666 302 660V361L303 61Q310 54 315 52T339 48T401 46H427V0H416Q395 3 257 3Q121 3 100 0H88V46H114Q136 46 152 46T177 47T193 50T201 52T207 57T213 61V578Z"></path><path stroke-width="1" id="E129-MJAMS-2A7D" d="M674 636Q682 636 688 630T694 615T687 601Q686 600 417 472L151 346L399 228Q687 92 691 87Q694 81 694 76Q694 58 676 56H670L382 192Q92 329 90 331Q83 336 83 348Q84 359 96 365Q104 369 382 500T665 634Q669 636 674 636ZM94 170Q102 172 104 172Q110 171 254 103T535 -30T678 -98Q694 -106 694 -118Q694 -136 676 -138H670L382 -2Q92 135 90 137Q83 142 83 154Q84 164 94 170Z"></path><path stroke-width="1" id="E129-MJMATHI-78" d="M52 289Q59 331 106 386T222 442Q257 442 286 424T329 379Q371 442 430 442Q467 442 494 420T522 361Q522 332 508 314T481 292T458 288Q439 288 427 299T415 328Q415 374 465 391Q454 404 425 404Q412 404 406 402Q368 386 350 336Q290 115 290 78Q290 50 306 38T341 26Q378 26 414 59T463 140Q466 150 469 151T485 153H489Q504 153 504 145Q504 144 502 134Q486 77 440 33T333 -11Q263 -11 227 52Q186 -10 133 -10H127Q78 -10 57 16T35 71Q35 103 54 123T99 143Q142 143 142 101Q142 81 130 66T107 46T94 41L91 40Q91 39 97 36T113 29T132 26Q168 26 194 71Q203 87 217 139T245 247T261 313Q266 340 266 352Q266 380 251 392T217 404Q177 404 142 372T93 290Q91 281 88 280T72 278H58Q52 284 52 289Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E129-MJMAIN-2212" x="0" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E129-MJMAIN-31" x="778" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E129-MJAMS-2A7D" x="1556" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E129-MJMATHI-78" x="2613" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E129-MJAMS-2A7D" x="3463" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E129-MJMAIN-31" x="4519" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-128">-1 \leqslant x \leqslant 1</script>，类似于 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-129-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="9.852ex" height="2.227ex" viewBox="0 -755.9 4241.6 958.9" role="img" focusable="false" style="vertical-align: -0.472ex;"><defs><path stroke-width="1" id="E130-MJMAIN-31" d="M213 578L200 573Q186 568 160 563T102 556H83V602H102Q149 604 189 617T245 641T273 663Q275 666 285 666Q294 666 302 660V361L303 61Q310 54 315 52T339 48T401 46H427V0H416Q395 3 257 3Q121 3 100 0H88V46H114Q136 46 152 46T177 47T193 50T201 52T207 57T213 61V578Z"></path><path stroke-width="1" id="E130-MJAMS-2A7D" d="M674 636Q682 636 688 630T694 615T687 601Q686 600 417 472L151 346L399 228Q687 92 691 87Q694 81 694 76Q694 58 676 56H670L382 192Q92 329 90 331Q83 336 83 348Q84 359 96 365Q104 369 382 500T665 634Q669 636 674 636ZM94 170Q102 172 104 172Q110 171 254 103T535 -30T678 -98Q694 -106 694 -118Q694 -136 676 -138H670L382 -2Q92 135 90 137Q83 142 83 154Q84 164 94 170Z"></path><path stroke-width="1" id="E130-MJMATHI-78" d="M52 289Q59 331 106 386T222 442Q257 442 286 424T329 379Q371 442 430 442Q467 442 494 420T522 361Q522 332 508 314T481 292T458 288Q439 288 427 299T415 328Q415 374 465 391Q454 404 425 404Q412 404 406 402Q368 386 350 336Q290 115 290 78Q290 50 306 38T341 26Q378 26 414 59T463 140Q466 150 469 151T485 153H489Q504 153 504 145Q504 144 502 134Q486 77 440 33T333 -11Q263 -11 227 52Q186 -10 133 -10H127Q78 -10 57 16T35 71Q35 103 54 123T99 143Q142 143 142 101Q142 81 130 66T107 46T94 41L91 40Q91 39 97 36T113 29T132 26Q168 26 194 71Q203 87 217 139T245 247T261 313Q266 340 266 352Q266 380 251 392T217 404Q177 404 142 372T93 290Q91 281 88 280T72 278H58Q52 284 52 289Z"></path><path stroke-width="1" id="E130-MJMAIN-33" d="M127 463Q100 463 85 480T69 524Q69 579 117 622T233 665Q268 665 277 664Q351 652 390 611T430 522Q430 470 396 421T302 350L299 348Q299 347 308 345T337 336T375 315Q457 262 457 175Q457 96 395 37T238 -22Q158 -22 100 21T42 130Q42 158 60 175T105 193Q133 193 151 175T169 130Q169 119 166 110T159 94T148 82T136 74T126 70T118 67L114 66Q165 21 238 21Q293 21 321 74Q338 107 338 175V195Q338 290 274 322Q259 328 213 329L171 330L168 332Q166 335 166 348Q166 366 174 366Q202 366 232 371Q266 376 294 413T322 525V533Q322 590 287 612Q265 626 240 626Q208 626 181 615T143 592T132 580H135Q138 579 143 578T153 573T165 566T175 555T183 540T186 520Q186 498 172 481T127 463Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E130-MJMAIN-31" x="0" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E130-MJAMS-2A7D" x="778" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E130-MJMATHI-78" x="1834" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E130-MJAMS-2A7D" x="2684" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E130-MJMAIN-33" x="3741" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-129">1\leqslant x \leqslant 3</script> 等也是可取的，而诸如 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-130-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="16.309ex" height="2.227ex" viewBox="0 -755.9 7022.1 958.9" role="img" focusable="false" style="vertical-align: -0.472ex;"><defs><path stroke-width="1" id="E131-MJMAIN-2212" d="M84 237T84 250T98 270H679Q694 262 694 250T679 230H98Q84 237 84 250Z"></path><path stroke-width="1" id="E131-MJMAIN-31" d="M213 578L200 573Q186 568 160 563T102 556H83V602H102Q149 604 189 617T245 641T273 663Q275 666 285 666Q294 666 302 660V361L303 61Q310 54 315 52T339 48T401 46H427V0H416Q395 3 257 3Q121 3 100 0H88V46H114Q136 46 152 46T177 47T193 50T201 52T207 57T213 61V578Z"></path><path stroke-width="1" id="E131-MJMAIN-30" d="M96 585Q152 666 249 666Q297 666 345 640T423 548Q460 465 460 320Q460 165 417 83Q397 41 362 16T301 -15T250 -22Q224 -22 198 -16T137 16T82 83Q39 165 39 320Q39 494 96 585ZM321 597Q291 629 250 629Q208 629 178 597Q153 571 145 525T137 333Q137 175 145 125T181 46Q209 16 250 16Q290 16 318 46Q347 76 354 130T362 333Q362 478 354 524T321 597Z"></path><path stroke-width="1" id="E131-MJAMS-2A7D" d="M674 636Q682 636 688 630T694 615T687 601Q686 600 417 472L151 346L399 228Q687 92 691 87Q694 81 694 76Q694 58 676 56H670L382 192Q92 329 90 331Q83 336 83 348Q84 359 96 365Q104 369 382 500T665 634Q669 636 674 636ZM94 170Q102 172 104 172Q110 171 254 103T535 -30T678 -98Q694 -106 694 -118Q694 -136 676 -138H670L382 -2Q92 135 90 137Q83 142 83 154Q84 164 94 170Z"></path><path stroke-width="1" id="E131-MJMATHI-78" d="M52 289Q59 331 106 386T222 442Q257 442 286 424T329 379Q371 442 430 442Q467 442 494 420T522 361Q522 332 508 314T481 292T458 288Q439 288 427 299T415 328Q415 374 465 391Q454 404 425 404Q412 404 406 402Q368 386 350 336Q290 115 290 78Q290 50 306 38T341 26Q378 26 414 59T463 140Q466 150 469 151T485 153H489Q504 153 504 145Q504 144 502 134Q486 77 440 33T333 -11Q263 -11 227 52Q186 -10 133 -10H127Q78 -10 57 16T35 71Q35 103 54 123T99 143Q142 143 142 101Q142 81 130 66T107 46T94 41L91 40Q91 39 97 36T113 29T132 26Q168 26 194 71Q203 87 217 139T245 247T261 313Q266 340 266 352Q266 380 251 392T217 404Q177 404 142 372T93 290Q91 281 88 280T72 278H58Q52 284 52 289Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E131-MJMAIN-2212" x="0" y="0"></use><g transform="translate(778,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E131-MJMAIN-31"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E131-MJMAIN-30" x="500" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E131-MJMAIN-30" x="1001" y="0"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E131-MJAMS-2A7D" x="2557" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E131-MJMATHI-78" x="3614" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E131-MJAMS-2A7D" x="4464" y="0"></use><g transform="translate(5520,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E131-MJMAIN-31"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E131-MJMAIN-30" x="500" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E131-MJMAIN-30" x="1001" y="0"></use></g></g></svg></span><script type="math/tex" id="MathJax-Element-130">-100 \leqslant x \leqslant 100 </script>，<span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-131-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="24.578ex" height="2.227ex" viewBox="0 -755.9 10582.1 958.9" role="img" focusable="false" style="vertical-align: -0.472ex;"><defs><path stroke-width="1" id="E132-MJMAIN-2212" d="M84 237T84 250T98 270H679Q694 262 694 250T679 230H98Q84 237 84 250Z"></path><path stroke-width="1" id="E132-MJMAIN-30" d="M96 585Q152 666 249 666Q297 666 345 640T423 548Q460 465 460 320Q460 165 417 83Q397 41 362 16T301 -15T250 -22Q224 -22 198 -16T137 16T82 83Q39 165 39 320Q39 494 96 585ZM321 597Q291 629 250 629Q208 629 178 597Q153 571 145 525T137 333Q137 175 145 125T181 46Q209 16 250 16Q290 16 318 46Q347 76 354 130T362 333Q362 478 354 524T321 597Z"></path><path stroke-width="1" id="E132-MJMAIN-2E" d="M78 60Q78 84 95 102T138 120Q162 120 180 104T199 61Q199 36 182 18T139 0T96 17T78 60Z"></path><path stroke-width="1" id="E132-MJMAIN-31" d="M213 578L200 573Q186 568 160 563T102 556H83V602H102Q149 604 189 617T245 641T273 663Q275 666 285 666Q294 666 302 660V361L303 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71Q35 103 54 123T99 143Q142 143 142 101Q142 81 130 66T107 46T94 41L91 40Q91 39 97 36T113 29T132 26Q168 26 194 71Q203 87 217 139T245 247T261 313Q266 340 266 352Q266 380 251 392T217 404Q177 404 142 372T93 290Q91 281 88 280T72 278H58Q52 284 52 289Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E132-MJMAIN-2212" x="0" y="0"></use><g transform="translate(778,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E132-MJMAIN-30"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E132-MJMAIN-2E" x="500" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E132-MJMAIN-30" x="779" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E132-MJMAIN-30" x="1279" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E132-MJMAIN-30" x="1780" y="0"></use><use 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y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E132-MJMAIN-30" x="2280" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E132-MJMAIN-31" x="2781" y="0"></use></g></g></svg></span><script type="math/tex" id="MathJax-Element-131">-0.00001 \leqslant x \leqslant 0.00001</script>，就显得过大/过小了。</p><p>另外注意，一旦采用特征缩放，我们就需对所有的输入采用特征缩放，包括训练集、测试集、预测输入等。</p><h2><a name='header-n75' class='md-header-anchor '></a>4.4 梯度下降实践2-学习速率(Gradient Descent in Practice II - Learning Rate)</h2><p>通常，有两种方法来确定函数是否收敛</p><ul><li><p>多次迭代收敛法</p><ul><li>无法确定需要多少次迭代</li><li>较易绘制关于迭代次数的图像</li><li>根据图像易预测所需的迭代次数</li></ul></li><li><p>自动化测试收敛法（比较阈值）</p><ul><li>不易选取阈值</li><li>损失函数近乎直线时无法确定收敛情况</li></ul></li></ul><p>对于梯度下降，一般采用多次迭代收敛法来得出最小化损失函数的参数值，自动化测试收敛法（如设定 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-132-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="12.514ex" 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xlink:href="#E133-MJMAIN-3C" x="2326" y="0"></use><g transform="translate(3382,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E133-MJMAIN-31"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E133-MJMAIN-30" x="500" y="0"></use><g transform="translate(1001,393)"><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E133-MJMAIN-2212" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E133-MJMAIN-33" x="778" y="0"></use></g></g></g></svg></span><script type="math/tex" id="MathJax-Element-132">J\left(\theta\right) < {10}^{-3}</script> 为阈值）则几乎不会被使用。</p><p>我们可以通过绘制<strong>损失函数关于迭代次数的图像</strong>，可视化梯度下降的执行过程，借助直观的图形来发现损失函数趋向于多少时能趋于收敛，依据图像变化情况，确定诸如学习速率的取值，迭代次数的大小等问题。</p><p><img src='image/20180108_103357.png' alt='' /></p><p>对于学习速率 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-71-Frame" tabindex="-1" style="font-size: 100%; display: 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id="MathJax-Element-71">\alpha</script>，一般上图展现的为适中情况，下图中，左图可能表明 <strong><span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-71-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="1.488ex" height="1.41ex" viewBox="0 -504.6 640.5 607.1" role="img" focusable="false" style="vertical-align: -0.238ex;"><defs><path stroke-width="1" id="E71-MJMATHI-3B1" d="M34 156Q34 270 120 356T309 442Q379 442 421 402T478 304Q484 275 485 237V208Q534 282 560 374Q564 388 566 390T582 393Q603 393 603 385Q603 376 594 346T558 261T497 161L486 147L487 123Q489 67 495 47T514 26Q528 28 540 37T557 60Q559 67 562 68T577 70Q597 70 597 62Q597 56 591 43Q579 19 556 5T512 -10H505Q438 -10 414 62L411 69L400 61Q390 53 370 41T325 18T267 -2T203 -11Q124 -11 79 39T34 156ZM208 26Q257 26 306 47T379 90L403 112Q401 255 396 290Q382 405 304 405Q235 405 183 332Q156 292 139 224T121 120Q121 71 146 49T208 26Z"></path></defs><g 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x="779" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E134-MJMAIN-33" x="1279" y="0"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E134-MJMAIN-2C" x="12073" y="0"></use><g transform="translate(12796,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E134-MJMAIN-30"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E134-MJMAIN-2E" x="500" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E134-MJMAIN-31" x="779" y="0"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E134-MJMAIN-2C" x="14075" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E134-MJMAIN-2026" x="14798" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-133">\dots\;0,001,\;0.003,\;0.01,\;0.03,\;0.1,\;\dots</script></p><h2><a name='header-n114' class='md-header-anchor '></a>4.5 特征和多项式回归(Features and Polynomial Regression)</h2><p>在特征选取时，我们也可以自己归纳总结，定义一个新的特征，用来<strong>取代或拆分</strong>旧的一个或多个特征。比如，对于房屋面积特征来说，我们可以将其拆分为长度和宽度两个特征，反之，我们也可以合并长度和宽度这两个特征为面积这一个特征。</p><p>线性回归只能以直线来对数据进行拟合，有时候需要使用<strong>曲线</strong>来对数据进行拟合，即<strong>多项式回归(Polynomial Regression)</strong>。</p><p>比如一个二次方模型：<span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-134-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="25.85ex" height="3.044ex" viewBox="0 -906.7 11129.6 1310.7" role="img" focusable="false" style="vertical-align: -0.938ex;"><defs><path stroke-width="1" id="E135-MJMATHI-68" d="M137 683Q138 683 209 688T282 694Q294 694 294 685Q294 674 258 534Q220 386 220 383Q220 381 227 388Q288 442 357 442Q411 442 444 415T478 336Q478 285 440 178T402 50Q403 36 407 31T422 26Q450 26 474 56T513 138Q516 149 519 151T535 153Q555 153 555 145Q555 144 551 130Q535 71 500 33Q466 -10 419 -10H414Q367 -10 346 17T325 74Q325 90 361 192T398 345Q398 404 354 404H349Q266 404 205 306L198 293L164 158Q132 28 127 16Q114 -11 83 -11Q69 -11 59 -2T48 16Q48 30 121 320L195 616Q195 629 188 632T149 637H128Q122 643 122 645T124 664Q129 683 137 683Z"></path><path stroke-width="1" id="E135-MJMATHI-3B8" d="M35 200Q35 302 74 415T180 610T319 704Q320 704 327 704T339 705Q393 701 423 656Q462 596 462 495Q462 380 417 261T302 66T168 -10H161Q125 -10 99 10T60 63T41 130T35 200ZM383 566Q383 668 330 668Q294 668 260 623T204 521T170 421T157 371Q206 370 254 370L351 371Q352 372 359 404T375 484T383 566ZM113 132Q113 26 166 26Q181 26 198 36T239 74T287 161T335 307L340 324H145Q145 321 136 286T120 208T113 132Z"></path><path stroke-width="1" id="E135-MJMAIN-28" d="M94 250Q94 319 104 381T127 488T164 576T202 643T244 695T277 729T302 750H315H319Q333 750 333 741Q333 738 316 720T275 667T226 581T184 443T167 250T184 58T225 -81T274 -167T316 -220T333 -241Q333 -250 318 -250H315H302L274 -226Q180 -141 137 -14T94 250Z"></path><path stroke-width="1" id="E135-MJMATHI-78" d="M52 289Q59 331 106 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y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E141-MJSZ1-29" x="2768" y="-1"></use><g transform="translate(3227,577)"><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E141-MJMAIN-2212" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E141-MJMAIN-31" x="778" y="0"></use></g></g><g transform="translate(6035,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E141-MJMATHI-58" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E141-MJMATHI-54" x="1215" y="513"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E141-MJMATHI-79" x="7493" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-140">\theta ={{\left( {X^T}X \right)}^{-1}}{X^{T}}y</script> ，Octave 中为 <code>theta = inv(X&#39;*X)*X&#39;*y</code>。</p><blockquote><p><span class="MathJax_Preview"></span><span 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x="1512" y="-1"></use></g></g></svg></span><script type="math/tex" id="MathJax-Element-145">O\left( {{n}^{3}} \right)</script></td></tr><tr><td>适用范围<sup><a href='#dfref-footnote-2' name='ref-footnote-2'>2</a></sup></td><td>各类模型</td><td>只适用线性模型，且矩阵需可逆</td></tr></tbody></table></figure><p><strong>正规方程法的推导过程</strong>：</p><p>​	<span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-147-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="40.88ex" height="17.752ex" viewBox="0 -4073 17600.8 7643.4" role="img" focusable="false" style="vertical-align: -8.293ex;"><defs><path stroke-width="1" id="E148-MJMATHI-4A" d="M447 625Q447 637 354 637H329Q323 642 323 645T325 664Q329 677 335 683H352Q393 681 498 681Q541 681 568 681T605 682T619 682Q633 682 633 672Q633 670 630 658Q626 642 623 640T604 637Q552 637 545 623Q541 610 483 376Q420 128 419 127Q397 64 333 21T195 -22Q137 -22 97 8T57 88Q57 130 80 152T132 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xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E149-MJSZ1-28"></use><g transform="translate(458,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E149-MJMATHI-3B8" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E149-MJMATHI-54" x="663" y="612"></use></g><g transform="translate(1526,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E149-MJMATHI-58" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E149-MJMATHI-54" x="1215" y="581"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E149-MJMATHI-58" x="2984" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E149-MJMATHI-3B8" x="3836" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E149-MJMAIN-2212" x="4528" y="0"></use><g transform="translate(5529,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E149-MJMATHI-3B8" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E149-MJMATHI-54" x="663" y="612"></use></g><g transform="translate(6596,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E149-MJMATHI-58" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E149-MJMATHI-54" x="1215" y="581"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E149-MJMATHI-79" x="8054" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E149-MJMAIN-2212" x="8774" y="0"></use><g transform="translate(9775,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E149-MJMATHI-79" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E149-MJMATHI-54" x="706" y="513"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E149-MJMATHI-58" x="10872" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E149-MJMATHI-3B8" x="11725" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E149-MJMAIN-2B" x="12417" y="0"></use><g transform="translate(13417,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E149-MJMATHI-79" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E149-MJMATHI-54" x="706" y="513"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E149-MJMATHI-79" x="14515" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E149-MJSZ1-29" x="15013" y="-1"></use></g></g></svg></span><script type="math/tex" id="MathJax-Element-148">J(\theta )= \frac{1}{2m}\left( {{\theta }^{T}}{{X}^{T}}X\theta -{{\theta}^{T}}{{X}^{T}}y-{{y}^{T}}X\theta + {{y}^{T}}y \right)</script></p><p>注意到 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-149-Frame" tabindex="-1" style="font-size: 100%; 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427 45T436 36Q436 31 433 22Q429 4 424 1L422 0Q419 0 415 0Q410 0 363 1T228 2Q99 2 64 0H49Q43 6 43 9T45 27Q49 40 55 46H83H94Q174 46 189 55Q190 56 191 56Q196 59 201 76T241 233Q258 301 269 344Q339 619 339 625Q339 630 310 630H279Q212 630 191 624Q146 614 121 583T67 467Q60 445 57 441T43 437H40Z"></path><path stroke-width="1" id="E150-MJMATHI-58" d="M42 0H40Q26 0 26 11Q26 15 29 27Q33 41 36 43T55 46Q141 49 190 98Q200 108 306 224T411 342Q302 620 297 625Q288 636 234 637H206Q200 643 200 645T202 664Q206 677 212 683H226Q260 681 347 681Q380 681 408 681T453 682T473 682Q490 682 490 671Q490 670 488 658Q484 643 481 640T465 637Q434 634 411 620L488 426L541 485Q646 598 646 610Q646 628 622 635Q617 635 609 637Q594 637 594 648Q594 650 596 664Q600 677 606 683H618Q619 683 643 683T697 681T738 680Q828 680 837 683H845Q852 676 852 672Q850 647 840 637H824Q790 636 763 628T722 611T698 593L687 584Q687 585 592 480L505 384Q505 383 536 304T601 142T638 56Q648 47 699 46Q734 46 734 37Q734 35 732 23Q728 7 725 4T711 1Q708 1 678 1T589 2Q528 2 496 2T461 1Q444 1 444 10Q444 11 446 25Q448 35 450 39T455 44T464 46T480 47T506 54Q523 62 523 64Q522 64 476 181L429 299Q241 95 236 84Q232 76 232 72Q232 53 261 47Q262 47 267 47T273 46Q276 46 277 46T280 45T283 42T284 35Q284 26 282 19Q279 6 276 4T261 1Q258 1 243 1T201 2T142 2Q64 2 42 0Z"></path><path stroke-width="1" id="E150-MJMATHI-79" d="M21 287Q21 301 36 335T84 406T158 442Q199 442 224 419T250 355Q248 336 247 334Q247 331 231 288T198 191T182 105Q182 62 196 45T238 27Q261 27 281 38T312 61T339 94Q339 95 344 114T358 173T377 247Q415 397 419 404Q432 431 462 431Q475 431 483 424T494 412T496 403Q496 390 447 193T391 -23Q363 -106 294 -155T156 -205Q111 -205 77 -183T43 -117Q43 -95 50 -80T69 -58T89 -48T106 -45Q150 -45 150 -87Q150 -107 138 -122T115 -142T102 -147L99 -148Q101 -153 118 -160T152 -167H160Q177 -167 186 -165Q219 -156 247 -127T290 -65T313 -9T321 21L315 17Q309 13 296 6T270 -6Q250 -11 231 -11Q185 -11 150 11T104 82Q103 89 103 113Q103 170 138 262T173 379Q173 380 173 381Q173 390 173 393T169 400T158 404H154Q131 404 112 385T82 344T65 302T57 280Q55 278 41 278H27Q21 284 21 287Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E150-MJMATHI-3B8" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E150-MJMATHI-54" x="663" y="612"></use><g transform="translate(1067,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E150-MJMATHI-58" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E150-MJMATHI-54" x="1215" y="581"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E150-MJMATHI-79" x="2525" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-149">{{\theta}^{T}}{{X}^{T}}y</script> 与 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-150-Frame" tabindex="-1" 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404T375 484T383 566ZM113 132Q113 26 166 26Q181 26 198 36T239 74T287 161T335 307L340 324H145Q145 321 136 286T120 208T113 132Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E151-MJMATHI-79" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E151-MJMATHI-54" x="706" y="513"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E151-MJMATHI-58" x="1097" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E151-MJMATHI-3B8" x="1950" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-150">{{y}^{T}}X\theta</script> 都为标量，实际上是等价的，则</p><p>​	<span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-151-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="35.891ex" 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xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E152-MJMATHI-79" x="11858" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E152-MJMAIN-2B" x="12578" y="0"></use><g transform="translate(13579,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E152-MJMATHI-79" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E152-MJMATHI-54" x="706" y="513"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E152-MJMATHI-79" x="14676" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E152-MJMAIN-5D" x="15174" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-151">J(\theta) = \frac{1}{2m}[X^TX\theta-2\theta^TX^Ty+y^Ty]</script></p><p>&nbsp;</p><p>接下来对<span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-152-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg 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37Q734 35 732 23Q728 7 725 4T711 1Q708 1 678 1T589 2Q528 2 496 2T461 1Q444 1 444 10Q444 11 446 25Q448 35 450 39T455 44T464 46T480 47T506 54Q523 62 523 64Q522 64 476 181L429 299Q241 95 236 84Q232 76 232 72Q232 53 261 47Q262 47 267 47T273 46Q276 46 277 46T280 45T283 42T284 35Q284 26 282 19Q279 6 276 4T261 1Q258 1 243 1T201 2T142 2Q64 2 42 0Z"></path><path stroke-width="1" id="E154-MJMATHI-54" d="M40 437Q21 437 21 445Q21 450 37 501T71 602L88 651Q93 669 101 677H569H659Q691 677 697 676T704 667Q704 661 687 553T668 444Q668 437 649 437Q640 437 637 437T631 442L629 445Q629 451 635 490T641 551Q641 586 628 604T573 629Q568 630 515 631Q469 631 457 630T439 622Q438 621 368 343T298 60Q298 48 386 46Q418 46 427 45T436 36Q436 31 433 22Q429 4 424 1L422 0Q419 0 415 0Q410 0 363 1T228 2Q99 2 64 0H49Q43 6 43 9T45 27Q49 40 55 46H83H94Q174 46 189 55Q190 56 191 56Q196 59 201 76T241 233Q258 301 269 344Q339 619 339 625Q339 630 310 630H279Q212 630 191 624Q146 614 121 583T67 467Q60 445 57 441T43 437H40Z"></path><path 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446 25Q448 35 450 39T455 44T464 46T480 47T506 54Q523 62 523 64Q522 64 476 181L429 299Q241 95 236 84Q232 76 232 72Q232 53 261 47Q262 47 267 47T273 46Q276 46 277 46T280 45T283 42T284 35Q284 26 282 19Q279 6 276 4T261 1Q258 1 243 1T201 2T142 2Q64 2 42 0Z"></path><path stroke-width="1" id="E155-MJMATHI-54" d="M40 437Q21 437 21 445Q21 450 37 501T71 602L88 651Q93 669 101 677H569H659Q691 677 697 676T704 667Q704 661 687 553T668 444Q668 437 649 437Q640 437 637 437T631 442L629 445Q629 451 635 490T641 551Q641 586 628 604T573 629Q568 630 515 631Q469 631 457 630T439 622Q438 621 368 343T298 60Q298 48 386 46Q418 46 427 45T436 36Q436 31 433 22Q429 4 424 1L422 0Q419 0 415 0Q410 0 363 1T228 2Q99 2 64 0H49Q43 6 43 9T45 27Q49 40 55 46H83H94Q174 46 189 55Q190 56 191 56Q196 59 201 76T241 233Q258 301 269 344Q339 619 339 625Q339 630 310 630H279Q212 630 191 624Q146 614 121 583T67 467Q60 445 57 441T43 437H40Z"></path><path stroke-width="1" id="E155-MJMATHI-41" d="M208 74Q208 50 254 46Q272 46 272 35Q272 34 270 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xlink:href="#E155-MJMATHI-64" x="0" y="0"></use><g transform="translate(370,0)"><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E155-MJMATHI-58" x="0" y="0"></use><use transform="scale(0.5)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E155-MJMATHI-54" x="1215" y="581"></use></g><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E155-MJMATHI-41" x="1981" y="0"></use></g><g transform="translate(539,-396)"><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E155-MJMATHI-64" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E155-MJMATHI-58" x="523" y="0"></use></g></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E155-MJMAIN-3D" x="2569" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E155-MJMATHI-41" x="3625" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-154">\frac{dX^TA}{dX}={A}</script></p><p>&nbsp;</p><p>所以有:</p><p>​	<span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-155-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="29.352ex" height="3.978ex" viewBox="0 -1208.2 12637.5 1712.8" role="img" focusable="false" style="vertical-align: -1.172ex;"><defs><path stroke-width="1" id="E156-MJMAIN-2202" d="M202 508Q179 508 169 520T158 547Q158 557 164 577T185 624T230 675T301 710L333 715H345Q378 715 384 714Q447 703 489 661T549 568T566 457Q566 362 519 240T402 53Q321 -22 223 -22Q123 -22 73 56Q42 102 42 148V159Q42 276 129 370T322 465Q383 465 414 434T455 367L458 378Q478 461 478 515Q478 603 437 639T344 676Q266 676 223 612Q264 606 264 572Q264 547 246 528T202 508ZM430 306Q430 372 401 400T333 428Q270 428 222 382Q197 354 183 323T150 221Q132 149 132 116Q132 21 232 21Q244 21 250 22Q327 35 374 112Q389 137 409 196T430 306Z"></path><path stroke-width="1" id="E156-MJMATHI-4A" d="M447 625Q447 637 354 637H329Q323 642 323 645T325 664Q329 677 335 683H352Q393 681 498 681Q541 681 568 681T605 682T619 682Q633 682 633 672Q633 670 630 658Q626 642 623 640T604 637Q552 637 545 623Q541 610 483 376Q420 128 419 127Q397 64 333 21T195 -22Q137 -22 97 8T57 88Q57 130 80 152T132 174Q177 174 182 130Q182 98 164 80T123 56Q115 54 115 53T122 44Q148 15 197 15Q235 15 271 47T324 130Q328 142 387 380T447 625Z"></path><path stroke-width="1" id="E156-MJMAIN-28" d="M94 250Q94 319 104 381T127 488T164 576T202 643T244 695T277 729T302 750H315H319Q333 750 333 741Q333 738 316 720T275 667T226 581T184 443T167 250T184 58T225 -81T274 -167T316 -220T333 -241Q333 -250 318 -250H315H302L274 -226Q180 -141 137 -14T94 250Z"></path><path stroke-width="1" id="E156-MJMATHI-3B8" d="M35 200Q35 302 74 415T180 610T319 704Q320 704 327 704T339 705Q393 701 423 656Q462 596 462 495Q462 380 417 261T302 66T168 -10H161Q125 -10 99 10T60 63T41 130T35 200ZM383 566Q383 668 330 668Q294 668 260 623T204 521T170 421T157 371Q206 370 254 370L351 371Q352 372 359 404T375 484T383 566ZM113 132Q113 26 166 26Q181 26 198 36T239 74T287 161T335 307L340 324H145Q145 321 136 286T120 208T113 132Z"></path><path stroke-width="1" id="E156-MJMAIN-29" d="M60 749L64 750Q69 750 74 750H86L114 726Q208 641 251 514T294 250Q294 182 284 119T261 12T224 -76T186 -143T145 -194T113 -227T90 -246Q87 -249 86 -250H74Q66 -250 63 -250T58 -247T55 -238Q56 -237 66 -225Q221 -64 221 250T66 725Q56 737 55 738Q55 746 60 749Z"></path><path stroke-width="1" id="E156-MJMAIN-3D" d="M56 347Q56 360 70 367H707Q722 359 722 347Q722 336 708 328L390 327H72Q56 332 56 347ZM56 153Q56 168 72 173H708Q722 163 722 153Q722 140 707 133H70Q56 140 56 153Z"></path><path stroke-width="1" id="E156-MJMAIN-31" d="M213 578L200 573Q186 568 160 563T102 556H83V602H102Q149 604 189 617T245 641T273 663Q275 666 285 666Q294 666 302 660V361L303 61Q310 54 315 52T339 48T401 46H427V0H416Q395 3 257 3Q121 3 100 0H88V46H114Q136 46 152 46T177 47T193 50T201 52T207 57T213 61V578Z"></path><path stroke-width="1" id="E156-MJMAIN-32" d="M109 429Q82 429 66 447T50 491Q50 562 103 614T235 666Q326 666 387 610T449 465Q449 422 429 383T381 315T301 241Q265 210 201 149L142 93L218 92Q375 92 385 97Q392 99 409 186V189H449V186Q448 183 436 95T421 3V0H50V19V31Q50 38 56 46T86 81Q115 113 136 137Q145 147 170 174T204 211T233 244T261 278T284 308T305 340T320 369T333 401T340 431T343 464Q343 527 309 573T212 619Q179 619 154 602T119 569T109 550Q109 549 114 549Q132 549 151 535T170 489Q170 464 154 447T109 429Z"></path><path stroke-width="1" id="E156-MJMATHI-6D" d="M21 287Q22 293 24 303T36 341T56 388T88 425T132 442T175 435T205 417T221 395T229 376L231 369Q231 367 232 367L243 378Q303 442 384 442Q401 442 415 440T441 433T460 423T475 411T485 398T493 385T497 373T500 364T502 357L510 367Q573 442 659 442Q713 442 746 415T780 336Q780 285 742 178T704 50Q705 36 709 31T724 26Q752 26 776 56T815 138Q818 149 821 151T837 153Q857 153 857 145Q857 144 853 130Q845 101 831 73T785 17T716 -10Q669 -10 648 17T627 73Q627 92 663 193T700 345Q700 404 656 404H651Q565 404 506 303L499 291L466 157Q433 26 428 16Q415 -11 385 -11Q372 -11 364 -4T353 8T350 18Q350 29 384 161L420 307Q423 322 423 345Q423 404 379 404H374Q288 404 229 303L222 291L189 157Q156 26 151 16Q138 -11 108 -11Q95 -11 87 -5T76 7T74 17Q74 30 112 181Q151 335 151 342Q154 357 154 369Q154 405 129 405Q107 405 92 377T69 316T57 280Q55 278 41 278H27Q21 284 21 287Z"></path><path stroke-width="1" id="E156-MJMATHI-58" d="M42 0H40Q26 0 26 11Q26 15 29 27Q33 41 36 43T55 46Q141 49 190 98Q200 108 306 224T411 342Q302 620 297 625Q288 636 234 637H206Q200 643 200 645T202 664Q206 677 212 683H226Q260 681 347 681Q380 681 408 681T453 682T473 682Q490 682 490 671Q490 670 488 658Q484 643 481 640T465 637Q434 634 411 620L488 426L541 485Q646 598 646 610Q646 628 622 635Q617 635 609 637Q594 637 594 648Q594 650 596 664Q600 677 606 683H618Q619 683 643 683T697 681T738 680Q828 680 837 683H845Q852 676 852 672Q850 647 840 637H824Q790 636 763 628T722 611T698 593L687 584Q687 585 592 480L505 384Q505 383 536 304T601 142T638 56Q648 47 699 46Q734 46 734 37Q734 35 732 23Q728 7 725 4T711 1Q708 1 678 1T589 2Q528 2 496 2T461 1Q444 1 444 10Q444 11 446 25Q448 35 450 39T455 44T464 46T480 47T506 54Q523 62 523 64Q522 64 476 181L429 299Q241 95 236 84Q232 76 232 72Q232 53 261 47Q262 47 267 47T273 46Q276 46 277 46T280 45T283 42T284 35Q284 26 282 19Q279 6 276 4T261 1Q258 1 243 1T201 2T142 2Q64 2 42 0Z"></path><path stroke-width="1" id="E156-MJMATHI-54" d="M40 437Q21 437 21 445Q21 450 37 501T71 602L88 651Q93 669 101 677H569H659Q691 677 697 676T704 667Q704 661 687 553T668 444Q668 437 649 437Q640 437 637 437T631 442L629 445Q629 451 635 490T641 551Q641 586 628 604T573 629Q568 630 515 631Q469 631 457 630T439 622Q438 621 368 343T298 60Q298 48 386 46Q418 46 427 45T436 36Q436 31 433 22Q429 4 424 1L422 0Q419 0 415 0Q410 0 363 1T228 2Q99 2 64 0H49Q43 6 43 9T45 27Q49 40 55 46H83H94Q174 46 189 55Q190 56 191 56Q196 59 201 76T241 233Q258 301 269 344Q339 619 339 625Q339 630 310 630H279Q212 630 191 624Q146 614 121 583T67 467Q60 445 57 441T43 437H40Z"></path><path stroke-width="1" id="E156-MJMAIN-2212" d="M84 237T84 250T98 270H679Q694 262 694 250T679 230H98Q84 237 84 250Z"></path><path stroke-width="1" id="E156-MJMATHI-79" d="M21 287Q21 301 36 335T84 406T158 442Q199 442 224 419T250 355Q248 336 247 334Q247 331 231 288T198 191T182 105Q182 62 196 45T238 27Q261 27 281 38T312 61T339 94Q339 95 344 114T358 173T377 247Q415 397 419 404Q432 431 462 431Q475 431 483 424T494 412T496 403Q496 390 447 193T391 -23Q363 -106 294 -155T156 -205Q111 -205 77 -183T43 -117Q43 -95 50 -80T69 -58T89 -48T106 -45Q150 -45 150 -87Q150 -107 138 -122T115 -142T102 -147L99 -148Q101 -153 118 -160T152 -167H160Q177 -167 186 -165Q219 -156 247 -127T290 -65T313 -9T321 21L315 17Q309 13 296 6T270 -6Q250 -11 231 -11Q185 -11 150 11T104 82Q103 89 103 113Q103 170 138 262T173 379Q173 380 173 381Q173 390 173 393T169 400T158 404H154Q131 404 112 385T82 344T65 302T57 280Q55 278 41 278H27Q21 284 21 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y="0"></use><g transform="translate(849,0)"><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E156-MJMAIN-28" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E156-MJMATHI-3B8" x="389" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E156-MJMAIN-29" x="859" y="0"></use></g></g><g transform="translate(559,-411)"><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E156-MJMAIN-2202" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E156-MJMATHI-3B8" x="567" y="0"></use></g></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E156-MJMAIN-3D" x="2369" y="0"></use><g transform="translate(3148,0)"><g transform="translate(397,0)"><rect stroke="none" width="1095" height="60" x="0" y="220"></rect><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E156-MJMAIN-31" x="524" y="572"></use><g transform="translate(60,-376)"><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E156-MJMAIN-32" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E156-MJMATHI-6D" x="500" y="0"></use></g></g></g><g transform="translate(4761,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E156-MJSZ1-28"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E156-MJMAIN-32" x="458" y="0"></use><g transform="translate(959,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E156-MJMATHI-58" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E156-MJMATHI-54" x="1215" y="581"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E156-MJMATHI-58" x="2416" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E156-MJMATHI-3B8" x="3269" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E156-MJMAIN-2212" x="3961" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E156-MJMAIN-32" x="4961" y="0"></use><g transform="translate(5462,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E156-MJMATHI-58" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E156-MJMATHI-54" x="1215" y="581"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E156-MJMATHI-79" x="6920" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E156-MJSZ1-29" x="7417" y="-1"></use></g></g></svg></span><script type="math/tex" id="MathJax-Element-155">\frac{\partial J\left( \theta  \right)}{\partial \theta }=\frac{1}{2m}\left(2{{X}^{T}}X\theta -2{{X}^{T}}y \right)</script></p><p>​	           <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-156-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="16.292ex" height="2.928ex" viewBox="0 -956.9 7014.5 1260.5" role="img" focusable="false" style="vertical-align: -0.705ex;"><defs><path stroke-width="1" id="E157-MJMAIN-3D" d="M56 347Q56 360 70 367H707Q722 359 722 347Q722 336 708 328L390 327H72Q56 332 56 347ZM56 153Q56 168 72 173H708Q722 163 722 153Q722 140 707 133H70Q56 140 56 153Z"></path><path stroke-width="1" id="E157-MJMATHI-58" d="M42 0H40Q26 0 26 11Q26 15 29 27Q33 41 36 43T55 46Q141 49 190 98Q200 108 306 224T411 342Q302 620 297 625Q288 636 234 637H206Q200 643 200 645T202 664Q206 677 212 683H226Q260 681 347 681Q380 681 408 681T453 682T473 682Q490 682 490 671Q490 670 488 658Q484 643 481 640T465 637Q434 634 411 620L488 426L541 485Q646 598 646 610Q646 628 622 635Q617 635 609 637Q594 637 594 648Q594 650 596 664Q600 677 606 683H618Q619 683 643 683T697 681T738 680Q828 680 837 683H845Q852 676 852 672Q850 647 840 637H824Q790 636 763 628T722 611T698 593L687 584Q687 585 592 480L505 384Q505 383 536 304T601 142T638 56Q648 47 699 46Q734 46 734 37Q734 35 732 23Q728 7 725 4T711 1Q708 1 678 1T589 2Q528 2 496 2T461 1Q444 1 444 10Q444 11 446 25Q448 35 450 39T455 44T464 46T480 47T506 54Q523 62 523 64Q522 64 476 181L429 299Q241 95 236 84Q232 76 232 72Q232 53 261 47Q262 47 267 47T273 46Q276 46 277 46T280 45T283 42T284 35Q284 26 282 19Q279 6 276 4T261 1Q258 1 243 1T201 2T142 2Q64 2 42 0Z"></path><path stroke-width="1" id="E157-MJMATHI-54" d="M40 437Q21 437 21 445Q21 450 37 501T71 602L88 651Q93 669 101 677H569H659Q691 677 697 676T704 667Q704 661 687 553T668 444Q668 437 649 437Q640 437 637 437T631 442L629 445Q629 451 635 490T641 551Q641 586 628 604T573 629Q568 630 515 631Q469 631 457 630T439 622Q438 621 368 343T298 60Q298 48 386 46Q418 46 427 45T436 36Q436 31 433 22Q429 4 424 1L422 0Q419 0 415 0Q410 0 363 1T228 2Q99 2 64 0H49Q43 6 43 9T45 27Q49 40 55 46H83H94Q174 46 189 55Q190 56 191 56Q196 59 201 76T241 233Q258 301 269 344Q339 619 339 625Q339 630 310 630H279Q212 630 191 624Q146 614 121 583T67 467Q60 445 57 441T43 437H40Z"></path><path stroke-width="1" id="E157-MJMATHI-3B8" d="M35 200Q35 302 74 415T180 610T319 704Q320 704 327 704T339 705Q393 701 423 656Q462 596 462 495Q462 380 417 261T302 66T168 -10H161Q125 -10 99 10T60 63T41 130T35 200ZM383 566Q383 668 330 668Q294 668 260 623T204 521T170 421T157 371Q206 370 254 370L351 371Q352 372 359 404T375 484T383 566ZM113 132Q113 26 166 26Q181 26 198 36T239 74T287 161T335 307L340 324H145Q145 321 136 286T120 208T113 132Z"></path><path stroke-width="1" id="E157-MJMAIN-2212" d="M84 237T84 250T98 270H679Q694 262 694 250T679 230H98Q84 237 84 250Z"></path><path stroke-width="1" id="E157-MJMATHI-79" d="M21 287Q21 301 36 335T84 406T158 442Q199 442 224 419T250 355Q248 336 247 334Q247 331 231 288T198 191T182 105Q182 62 196 45T238 27Q261 27 281 38T312 61T339 94Q339 95 344 114T358 173T377 247Q415 397 419 404Q432 431 462 431Q475 431 483 424T494 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y="-213"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E159-MJMAIN-2217" x="3609" y="0"></use><g transform="translate(4331,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E159-MJMAIN-28" x="0" y="0"></use><g transform="translate(389,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E159-MJMAIN-33"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E159-MJMAIN-2E" x="500" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E159-MJMAIN-32" x="779" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E159-MJMAIN-38" x="1279" y="0"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E159-MJMAIN-29" x="2169" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E159-MJMAIN-32" x="3618" y="675"></use></g></g></svg></span><script type="math/tex" id="MathJax-Element-158">{x_{1}}={x_{2}}*{{\left( 3.28 \right)}^{2}}</script>。</p></li><li><p>特征数量大于训练集的数量<span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-159-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="8.343ex" height="2.577ex" viewBox="0 -806.1 3592.1 1109.7" role="img" focusable="false" style="vertical-align: -0.705ex;"><defs><path stroke-width="1" id="E160-MJMAIN-28" d="M94 250Q94 319 104 381T127 488T164 576T202 643T244 695T277 729T302 750H315H319Q333 750 333 741Q333 738 316 720T275 667T226 581T184 443T167 250T184 58T225 -81T274 -167T316 -220T333 -241Q333 -250 318 -250H315H302L274 -226Q180 -141 137 -14T94 250Z"></path><path stroke-width="1" id="E160-MJMATHI-6D" d="M21 287Q22 293 24 303T36 341T56 388T88 425T132 442T175 435T205 417T221 395T229 376L231 369Q231 367 232 367L243 378Q303 442 384 442Q401 442 415 440T441 433T460 423T475 411T485 398T493 385T497 373T500 364T502 357L510 367Q573 442 659 442Q713 442 746 415T780 336Q780 285 742 178T704 50Q705 36 709 31T724 26Q752 26 776 56T815 138Q818 149 821 151T837 153Q857 153 857 145Q857 144 853 130Q845 101 831 73T785 17T716 -10Q669 -10 648 17T627 73Q627 92 663 193T700 345Q700 404 656 404H651Q565 404 506 303L499 291L466 157Q433 26 428 16Q415 -11 385 -11Q372 -11 364 -4T353 8T350 18Q350 29 384 161L420 307Q423 322 423 345Q423 404 379 404H374Q288 404 229 303L222 291L189 157Q156 26 151 16Q138 -11 108 -11Q95 -11 87 -5T76 7T74 17Q74 30 112 181Q151 335 151 342Q154 357 154 369Q154 405 129 405Q107 405 92 377T69 316T57 280Q55 278 41 278H27Q21 284 21 287Z"></path><path stroke-width="1" id="E160-MJAMS-2A7D" d="M674 636Q682 636 688 630T694 615T687 601Q686 600 417 472L151 346L399 228Q687 92 691 87Q694 81 694 76Q694 58 676 56H670L382 192Q92 329 90 331Q83 336 83 348Q84 359 96 365Q104 369 382 500T665 634Q669 636 674 636ZM94 170Q102 172 104 172Q110 171 254 103T535 -30T678 -98Q694 -106 694 -118Q694 -136 676 -138H670L382 -2Q92 135 90 137Q83 142 83 154Q84 164 94 170Z"></path><path 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-1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E160-MJMAIN-28" x="0" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E160-MJMATHI-6D" x="389" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E160-MJAMS-2A7D" x="1545" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E160-MJMATHI-6E" x="2602" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E160-MJMAIN-29" x="3202" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-159">\left(m \leqslant n \right)</script>。</p></li></ul><p>如果发现 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-160-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="5.366ex" height="2.344ex" viewBox="0 -906.7 2310.4 1009.2" role="img" focusable="false" style="vertical-align: -0.238ex;"><defs><path stroke-width="1" id="E161-MJMATHI-58" d="M42 0H40Q26 0 26 11Q26 15 29 27Q33 41 36 43T55 46Q141 49 190 98Q200 108 306 224T411 342Q302 620 297 625Q288 636 234 637H206Q200 643 200 645T202 664Q206 677 212 683H226Q260 681 347 681Q380 681 408 681T453 682T473 682Q490 682 490 671Q490 670 488 658Q484 643 481 640T465 637Q434 634 411 620L488 426L541 485Q646 598 646 610Q646 628 622 635Q617 635 609 637Q594 637 594 648Q594 650 596 664Q600 677 606 683H618Q619 683 643 683T697 681T738 680Q828 680 837 683H845Q852 676 852 672Q850 647 840 637H824Q790 636 763 628T722 611T698 593L687 584Q687 585 592 480L505 384Q505 383 536 304T601 142T638 56Q648 47 699 46Q734 46 734 37Q734 35 732 23Q728 7 725 4T711 1Q708 1 678 1T589 2Q528 2 496 2T461 1Q444 1 444 10Q444 11 446 25Q448 35 450 39T455 44T464 46T480 47T506 54Q523 62 523 64Q522 64 476 181L429 299Q241 95 236 84Q232 76 232 72Q232 53 261 47Q262 47 267 47T273 46Q276 46 277 46T280 45T283 42T284 35Q284 26 282 19Q279 6 276 4T261 1Q258 1 243 1T201 2T142 2Q64 2 42 0Z"></path><path stroke-width="1" id="E161-MJMATHI-54" d="M40 437Q21 437 21 445Q21 450 37 501T71 602L88 651Q93 669 101 677H569H659Q691 677 697 676T704 667Q704 661 687 553T668 444Q668 437 649 437Q640 437 637 437T631 442L629 445Q629 451 635 490T641 551Q641 586 628 604T573 629Q568 630 515 631Q469 631 457 630T439 622Q438 621 368 343T298 60Q298 48 386 46Q418 46 427 45T436 36Q436 31 433 22Q429 4 424 1L422 0Q419 0 415 0Q410 0 363 1T228 2Q99 2 64 0H49Q43 6 43 9T45 27Q49 40 55 46H83H94Q174 46 189 55Q190 56 191 56Q196 59 201 76T241 233Q258 301 269 344Q339 619 339 625Q339 630 310 630H279Q212 630 191 624Q146 614 121 583T67 467Q60 445 57 441T43 437H40Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E161-MJMATHI-58" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E161-MJMATHI-54" x="1215" y="513"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E161-MJMATHI-58" x="1457" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-160">X^TX</script> 的结果不可逆，可尝试：</p><ul><li>减少多余/重复特征</li><li>增加训练集数量</li><li>使用正则化（后文）</li></ul><p>对于这类不可逆的矩阵，我们称之为<strong>奇异矩阵</strong>或<strong>退化矩阵</strong>。</p><p>这种情况下，如果还想使用正规方程法，在Octave中，可以选用 <code>pinv</code> 函数，<code>pinv</code> 区别于 <code>inv</code>，<code>pinv</code> 函数被称为伪逆函数，在矩阵不可逆的时候，使用这个函数仍可正确地计算出 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-69-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="1.09ex" height="2.11ex" viewBox="0 -806.1 469.5 908.7" role="img" focusable="false" style="vertical-align: -0.238ex;"><defs><path stroke-width="1" id="E69-MJMATHI-3B8" d="M35 200Q35 302 74 415T180 610T319 704Q320 704 327 704T339 705Q393 701 423 656Q462 596 462 495Q462 380 417 261T302 66T168 -10H161Q125 -10 99 10T60 63T41 130T35 200ZM383 566Q383 668 330 668Q294 668 260 623T204 521T170 421T157 371Q206 370 254 370L351 371Q352 372 359 404T375 484T383 566ZM113 132Q113 26 166 26Q181 26 198 36T239 74T287 161T335 307L340 324H145Q145 321 136 286T120 208T113 132Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E69-MJMATHI-3B8" x="0" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-69">\theta</script> 的值。</p><h1><a name='header-n226' class='md-header-anchor '></a>5 Octave/Matlab Tutorial</h1><p>复习时可直接倍速回顾视频，笔记整理暂留。</p><h2><a name='header-n229' class='md-header-anchor '></a>5.1 Basic Operations</h2><h2><a name='header-n230' class='md-header-anchor '></a>5.2 Moving Data Around</h2><h2><a name='header-n231' class='md-header-anchor '></a>5.3 Computing on Data</h2><h2><a name='header-n232' class='md-header-anchor '></a>5.4 Plotting Data</h2><h2><a name='header-n233' class='md-header-anchor '></a>5.5 Control Statements_ for, while, if statement</h2><h2><a name='header-n234' class='md-header-anchor '></a>5.6 Vectorization</h2><h2><a name='header-n235' class='md-header-anchor '></a>5.x 常用函数整理</h2><div class='footnotes-area'  ><hr/>
-<div class='footnote-line'><span class='md-fn-count'>1</span> 一般来说，当 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-146-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="1.395ex" height="1.438ex" viewBox="0 -505.2 600.5 619.1" role="img" focusable="false" style="vertical-align: -0.264ex;"><defs><path stroke-width="1" id="E147-MJMATHI-6E" d="M21 287Q22 293 24 303T36 341T56 388T89 425T135 442Q171 442 195 424T225 390T231 369Q231 367 232 367L243 378Q304 442 382 442Q436 442 469 415T503 336T465 179T427 52Q427 26 444 26Q450 26 453 27Q482 32 505 65T540 145Q542 153 560 153Q580 153 580 145Q580 144 576 130Q568 101 554 73T508 17T439 -10Q392 -10 371 17T350 73Q350 92 386 193T423 345Q423 404 379 404H374Q288 404 229 303L222 291L189 157Q156 26 151 16Q138 -11 108 -11Q95 -11 87 -5T76 7T74 17Q74 30 112 180T152 343Q153 348 153 366Q153 405 129 405Q91 405 66 305Q60 285 60 284Q58 278 41 278H27Q21 284 21 287Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E147-MJMATHI-6E" x="0" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-146">n</script> 超过 10000 时，对于正规方程而言，特征量较大。<a name='dfref-footnote-1' href='#ref-footnote-1' title='回到文档' class='reversefootnote' >↩</a></div>
+<div  id='write'  class = 'is-node show-fences-line-number'><div class='md-toc' mdtype='toc'><p class="md-toc-content"><span class="md-toc-item md-toc-h1" data-ref="n2"><a class="md-toc-inner" href="#header-n2">4 多变量线性回归(Linear Regression with Multiple Variables)</a></span><span class="md-toc-item md-toc-h2" data-ref="n3"><a class="md-toc-inner" href="#header-n3">4.1 多特征(Multiple Features)</a></span><span class="md-toc-item md-toc-h2" data-ref="n36"><a class="md-toc-inner" href="#header-n36">4.2 多变量梯度下降(Gradient Descent for Multiple Variables)</a></span><span class="md-toc-item md-toc-h2" data-ref="n61"><a class="md-toc-inner" href="#header-n61">4.3 梯度下降实践1-特征值缩放(Gradient Descent in Practice I - Feature Scaling)</a></span><span class="md-toc-item md-toc-h2" data-ref="n82"><a class="md-toc-inner" href="#header-n82">4.4 梯度下降实践2-学习速率(Gradient Descent in Practice II - Learning Rate)</a></span><span class="md-toc-item md-toc-h2" data-ref="n121"><a class="md-toc-inner" href="#header-n121">4.5 特征和多项式回归(Features and Polynomial Regression)</a></span><span class="md-toc-item md-toc-h2" data-ref="n136"><a class="md-toc-inner" href="#header-n136">4.6 正规方程(Normal Equation)</a></span><span class="md-toc-item md-toc-h2" data-ref="n197"><a class="md-toc-inner" href="#header-n197">4.7 不可逆性正规方程(Normal Equation Noninvertibility)</a></span><span class="md-toc-item md-toc-h1" data-ref="n229"><a class="md-toc-inner" href="#header-n229">5 Octave/Matlab Tutorial</a></span><span class="md-toc-item md-toc-h2" data-ref="n232"><a class="md-toc-inner" href="#header-n232">5.1 Basic Operations</a></span><span class="md-toc-item md-toc-h2" data-ref="n233"><a class="md-toc-inner" href="#header-n233">5.2 Moving Data Around</a></span><span class="md-toc-item md-toc-h2" data-ref="n234"><a class="md-toc-inner" href="#header-n234">5.3 Computing on Data</a></span><span class="md-toc-item md-toc-h2" data-ref="n235"><a class="md-toc-inner" href="#header-n235">5.4 Plotting Data</a></span><span class="md-toc-item md-toc-h2" data-ref="n236"><a class="md-toc-inner" href="#header-n236">5.5 Control Statements_ for, while, if statement</a></span><span class="md-toc-item md-toc-h2" data-ref="n237"><a class="md-toc-inner" href="#header-n237">5.6 Vectorization</a></span><span class="md-toc-item md-toc-h2" data-ref="n238"><a class="md-toc-inner" href="#header-n238">5.x 常用函数整理</a></span></p></div><h1><a name='header-n2' class='md-header-anchor '></a>4 多变量线性回归(Linear Regression with Multiple Variables)</h1><h2><a name='header-n3' class='md-header-anchor '></a>4.1 多特征(Multiple Features)</h2><p>对于一个要度量的对象，一般来说会有不同维度的多个特征。比如之前的房屋价格预测例子中，除了房屋的面积大小，可能还有房屋的年限、房屋的层数等等其他特征：</p><p><img src='image/20180107_234509.png' alt='' /></p><p>这里由于特征不再只有一个，引入一些新的记号</p><blockquote><p><span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-149-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" 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id="MathJax-Element-149">n</script>: 特征的总数 </p><p> <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-93-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="3.409ex" height="2.461ex" viewBox="0 -956.9 1467.6 1059.4" role="img" focusable="false" style="vertical-align: -0.238ex;"><defs><path stroke-width="1" id="E94-MJMATHI-78" d="M52 289Q59 331 106 386T222 442Q257 442 286 424T329 379Q371 442 430 442Q467 442 494 420T522 361Q522 332 508 314T481 292T458 288Q439 288 427 299T415 328Q415 374 465 391Q454 404 425 404Q412 404 406 402Q368 386 350 336Q290 115 290 78Q290 50 306 38T341 26Q378 26 414 59T463 140Q466 150 469 151T485 153H489Q504 153 504 145Q504 144 502 134Q486 77 440 33T333 -11Q263 -11 227 52Q186 -10 133 -10H127Q78 -10 57 16T35 71Q35 103 54 123T99 143Q142 143 142 101Q142 81 130 66T107 46T94 41L91 40Q91 39 97 36T113 29T132 26Q168 26 194 71Q203 87 217 139T245 247T261 313Q266 340 266 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transform="translate(0,-3446.188741721854) scale(1,2.8112582781456954)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E102-MJSZ4-23A5"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E102-MJSZ4-23A6" x="0" y="-4555"></use></g></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E102-MJMAIN-2C" x="6345" y="0"></use><g transform="translate(6790,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E102-MJMATHI-78" x="0" y="0"></use><g transform="translate(572,521)"><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E102-MJMAIN-28" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E102-MJMAIN-32" x="389" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E102-MJMAIN-29" x="890" y="0"></use></g><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E102-MJMAIN-31" x="809" y="-435"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E102-MJMAIN-3D" x="8645" y="0"></use><g transform="translate(9701,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E102-MJMAIN-31"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E102-MJMAIN-34" x="500" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E102-MJMAIN-31" x="1001" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E102-MJMAIN-36" x="1501" y="0"></use></g></g></svg></span><script type="math/tex" id="MathJax-Element-101">{x}^{(2)}\text{=}\begin{bmatrix} 1416\\\ 3\\\ 2\\\ 40 \end{bmatrix}, {x}^{(2)}_{1} = 1416</script></p><p>多变量假设函数 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-108-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="1.339ex" height="1.994ex" viewBox="0 -755.9 576.5 858.4" role="img" focusable="false" style="vertical-align: -0.238ex;"><defs><path stroke-width="1" id="E109-MJMATHI-68" d="M137 683Q138 683 209 688T282 694Q294 694 294 685Q294 674 258 534Q220 386 220 383Q220 381 227 388Q288 442 357 442Q411 442 444 415T478 336Q478 285 440 178T402 50Q403 36 407 31T422 26Q450 26 474 56T513 138Q516 149 519 151T535 153Q555 153 555 145Q555 144 551 130Q535 71 500 33Q466 -10 419 -10H414Q367 -10 346 17T325 74Q325 90 361 192T398 345Q398 404 354 404H349Q266 404 205 306L198 293L164 158Q132 28 127 16Q114 -11 83 -11Q69 -11 59 -2T48 16Q48 30 121 320L195 616Q195 629 188 632T149 637H128Q122 643 122 645T124 664Q129 683 137 683Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E109-MJMATHI-68" x="0" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-108">h</script> 表示为：<span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-103-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="37.425ex" height="2.577ex" viewBox="0 -806.1 16113.4 1109.7" role="img" focusable="false" style="vertical-align: -0.705ex;"><defs><path stroke-width="1" id="E104-MJMATHI-68" d="M137 683Q138 683 209 688T282 694Q294 694 294 685Q294 674 258 534Q220 386 220 383Q220 381 227 388Q288 442 357 442Q411 442 444 415T478 336Q478 285 440 178T402 50Q403 36 407 31T422 26Q450 26 474 56T513 138Q516 149 519 151T535 153Q555 153 555 145Q555 144 551 130Q535 71 500 33Q466 -10 419 -10H414Q367 -10 346 17T325 74Q325 90 361 192T398 345Q398 404 354 404H349Q266 404 205 306L198 293L164 158Q132 28 127 16Q114 -11 83 -11Q69 -11 59 -2T48 16Q48 30 121 320L195 616Q195 629 188 632T149 637H128Q122 643 122 645T124 664Q129 683 137 683Z"></path><path stroke-width="1" id="E104-MJMATHI-3B8" d="M35 200Q35 302 74 415T180 610T319 704Q320 704 327 704T339 705Q393 701 423 656Q462 596 462 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x="663" y="-213"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E104-MJMAIN-2B" x="5006" y="0"></use><g transform="translate(6007,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E104-MJMATHI-3B8" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E104-MJMAIN-31" x="663" y="-213"></use></g><g transform="translate(6930,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E104-MJMATHI-78" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E104-MJMAIN-31" x="809" y="-213"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E104-MJMAIN-2B" x="8179" y="0"></use><g transform="translate(9179,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E104-MJMATHI-3B8" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E104-MJMAIN-32" x="663" y="-213"></use></g><g transform="translate(10103,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E104-MJMATHI-78" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E104-MJMAIN-32" x="809" y="-213"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E104-MJMAIN-2B" x="11129" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E104-MJMAIN-2E" x="11908" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E104-MJMAIN-2E" x="12353" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E104-MJMAIN-2E" x="12798" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E104-MJMAIN-2B" x="13243" y="0"></use><g transform="translate(14022,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E104-MJMATHI-3B8" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E104-MJMATHI-6E" x="663" y="-213"></use></g><g transform="translate(15016,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E104-MJMATHI-78" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E104-MJMATHI-6E" x="809" y="-213"></use></g></g></svg></span><script type="math/tex" id="MathJax-Element-103">h_{\theta}\left( x \right)={\theta_{0}}+{\theta_{1}}{x_{1}}+{\theta_{2}}{x_{2}}+...+{\theta_{n}}{x_{n}}</script></p><p>对于 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-104-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="2.145ex" height="2.461ex" viewBox="0 -806.1 923.4 1059.4" role="img" focusable="false" style="vertical-align: -0.588ex;"><defs><path stroke-width="1" id="E105-MJMATHI-3B8" d="M35 200Q35 302 74 415T180 610T319 704Q320 704 327 704T339 705Q393 701 423 656Q462 596 462 495Q462 380 417 261T302 66T168 -10H161Q125 -10 99 10T60 63T41 130T35 200ZM383 566Q383 668 330 668Q294 668 260 623T204 521T170 421T157 371Q206 370 254 370L351 371Q352 372 359 404T375 484T383 566ZM113 132Q113 26 166 26Q181 26 198 36T239 74T287 161T335 307L340 324H145Q145 321 136 286T120 208T113 132Z"></path><path stroke-width="1" id="E105-MJMAIN-30" d="M96 585Q152 666 249 666Q297 666 345 640T423 548Q460 465 460 320Q460 165 417 83Q397 41 362 16T301 -15T250 -22Q224 -22 198 -16T137 16T82 83Q39 165 39 320Q39 494 96 585ZM321 597Q291 629 250 629Q208 629 178 597Q153 571 145 525T137 333Q137 175 145 125T181 46Q209 16 250 16Q290 16 318 46Q347 76 354 130T362 333Q362 478 354 524T321 597Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E105-MJMATHI-3B8" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E105-MJMAIN-30" x="663" y="-213"></use></g></svg></span><script type="math/tex" id="MathJax-Element-104">\theta_0</script>，和单特征中一样，我们将其看作基础数值。例如，房价的基础价格。</p><p>参数向量的维度为 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-111-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="5.398ex" height="2.11ex" viewBox="0 -755.9 2323.9 908.7" role="img" focusable="false" style="vertical-align: -0.355ex;"><defs><path stroke-width="1" id="E112-MJMATHI-6E" d="M21 287Q22 293 24 303T36 341T56 388T89 425T135 442Q171 442 195 424T225 390T231 369Q231 367 232 367L243 378Q304 442 382 442Q436 442 469 415T503 336T465 179T427 52Q427 26 444 26Q450 26 453 27Q482 32 505 65T540 145Q542 153 560 153Q580 153 580 145Q580 144 576 130Q568 101 554 73T508 17T439 -10Q392 -10 371 17T350 73Q350 92 386 193T423 345Q423 404 379 404H374Q288 404 229 303L222 291L189 157Q156 26 151 16Q138 -11 108 -11Q95 -11 87 -5T76 7T74 17Q74 30 112 180T152 343Q153 348 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x="1823" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-111">n+1</script>，在特征向量中添加 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-106-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="2.384ex" height="1.76ex" viewBox="0 -504.6 1026.4 757.9" role="img" focusable="false" style="vertical-align: -0.588ex;"><defs><path stroke-width="1" id="E107-MJMATHI-78" d="M52 289Q59 331 106 386T222 442Q257 442 286 424T329 379Q371 442 430 442Q467 442 494 420T522 361Q522 332 508 314T481 292T458 288Q439 288 427 299T415 328Q415 374 465 391Q454 404 425 404Q412 404 406 402Q368 386 350 336Q290 115 290 78Q290 50 306 38T341 26Q378 26 414 59T463 140Q466 150 469 151T485 153H489Q504 153 504 145Q504 144 502 134Q486 77 440 33T333 -11Q263 -11 227 52Q186 -10 133 -10H127Q78 -10 57 16T35 71Q35 103 54 123T99 143Q142 143 142 101Q142 81 130 66T107 46T94 41L91 40Q91 39 97 36T113 29T132 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transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E68-MJMAIN-29" x="734" y="0"></use></g></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E68-MJMAIN-29" x="11303" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E68-MJMAIN-2212" x="11914" y="0"></use><g transform="translate(12915,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E68-MJMATHI-79" x="0" y="0"></use><g transform="translate(499,412)"><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E68-MJMAIN-28" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E68-MJMATHI-69" x="389" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E68-MJMAIN-29" x="734" y="0"></use></g></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E68-MJMAIN-29" x="14310" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" 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(h_\theta(x^{(i)}) - y^{(i)}) \cdot x_2^{(i)} \newline & \vdots \newline \; & \theta_n := \theta_n - \alpha \frac{1}{m} \sum\limits_{i=1}^{m} (h_\theta(x^{(i)}) - y^{(i)}) \cdot x_n^{(i)} &\newline \rbrace \end{aligned}</script></p><p>当然，同单变量梯度下降一样，计算时需要<strong>同时更新</strong>所有参数。</p><p><span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-118-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="12.776ex" height="2.811ex" viewBox="0 -906.7 5500.9 1210.2" role="img" focusable="false" style="vertical-align: -0.705ex;"><defs><path stroke-width="1" id="E119-MJMATHI-68" d="M137 683Q138 683 209 688T282 694Q294 694 294 685Q294 674 258 534Q220 386 220 383Q220 381 227 388Q288 442 357 442Q411 442 444 415T478 336Q478 285 440 178T402 50Q403 36 407 31T422 26Q450 26 474 56T513 138Q516 149 519 151T535 153Q555 153 555 145Q555 144 551 130Q535 71 500 33Q466 -10 419 -10H414Q367 -10 346 17T325 74Q325 90 361 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class='md-header-anchor '></a>4.3 梯度下降实践1-特征值缩放(Gradient Descent in Practice I - Feature Scaling)</h2><p>在应用梯度下降算法实践时，由于各特征值的范围不一，可能会影响损失函数收敛速度。</p><p>以房价预测问题为例，这里选取房屋面积大小和房间数量这两个特征。</p><p>下图中，左图是以原始数据绘制的损失函数轮廓图，右图为采用特征缩放（都除以最大值）后图像。左图中呈现的图像较扁，相对于使用特征缩放方法的右图，梯度下降算法需要更多次的迭代。</p><p><img src='image/20180108_100751.png' alt='' /></p><p>&nbsp;</p><p>为了优化梯度下降的收敛速度，采用特征缩放的技巧，使各特征值的<strong>范围尽量一致</strong>。</p><p>除了以上图人工选择并除以一个参数的方式，<strong>均值归一化(Mean normalization)</strong>方法更为便捷，可采用它来对所有特征值统一缩放：</p><p> <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-122-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="31.999ex" height="4.445ex" viewBox="0 -1208.2 13777.2 1913.9" role="img" focusable="false" style="vertical-align: -1.639ex;"><defs><path stroke-width="1" id="E123-MJMATHI-78" d="M52 289Q59 331 106 386T222 442Q257 442 286 424T329 379Q371 442 430 442Q467 442 494 420T522 361Q522 332 508 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119 166 110T159 94T148 82T136 74T126 70T118 67L114 66Q165 21 238 21Q293 21 321 74Q338 107 338 175V195Q338 290 274 322Q259 328 213 329L171 330L168 332Q166 335 166 348Q166 366 174 366Q202 366 232 371Q266 376 294 413T322 525V533Q322 590 287 612Q265 626 240 626Q208 626 181 615T143 592T132 580H135Q138 579 143 578T153 573T165 566T175 555T183 540T186 520Q186 498 172 481T127 463Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E126-MJMAIN-31" x="0" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E126-MJAMS-2A7D" x="778" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E126-MJMATHI-78" x="1834" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E126-MJAMS-2A7D" x="2684" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E126-MJMAIN-33" x="3741" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-125">1\leqslant x \leqslant 3</script> 等也是可取的，而诸如 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-126-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="16.309ex" height="2.227ex" viewBox="0 -755.9 7022.1 958.9" role="img" focusable="false" style="vertical-align: -0.472ex;"><defs><path stroke-width="1" id="E127-MJMAIN-2212" d="M84 237T84 250T98 270H679Q694 262 694 250T679 230H98Q84 237 84 250Z"></path><path stroke-width="1" id="E127-MJMAIN-31" d="M213 578L200 573Q186 568 160 563T102 556H83V602H102Q149 604 189 617T245 641T273 663Q275 666 285 666Q294 666 302 660V361L303 61Q310 54 315 52T339 48T401 46H427V0H416Q395 3 257 3Q121 3 100 0H88V46H114Q136 46 152 46T177 47T193 50T201 52T207 57T213 61V578Z"></path><path stroke-width="1" id="E127-MJMAIN-30" d="M96 585Q152 666 249 666Q297 666 345 640T423 548Q460 465 460 320Q460 165 417 83Q397 41 362 16T301 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153H489Q504 153 504 145Q504 144 502 134Q486 77 440 33T333 -11Q263 -11 227 52Q186 -10 133 -10H127Q78 -10 57 16T35 71Q35 103 54 123T99 143Q142 143 142 101Q142 81 130 66T107 46T94 41L91 40Q91 39 97 36T113 29T132 26Q168 26 194 71Q203 87 217 139T245 247T261 313Q266 340 266 352Q266 380 251 392T217 404Q177 404 142 372T93 290Q91 281 88 280T72 278H58Q52 284 52 289Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E127-MJMAIN-2212" x="0" y="0"></use><g transform="translate(778,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E127-MJMAIN-31"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E127-MJMAIN-30" x="500" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E127-MJMAIN-30" x="1001" y="0"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E127-MJAMS-2A7D" x="2557" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E127-MJMATHI-78" x="3614" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E127-MJAMS-2A7D" x="4464" y="0"></use><g transform="translate(5520,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E127-MJMAIN-31"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E127-MJMAIN-30" x="500" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E127-MJMAIN-30" x="1001" y="0"></use></g></g></svg></span><script type="math/tex" id="MathJax-Element-126">-100 \leqslant x \leqslant 100 </script>，<span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-127-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="24.578ex" height="2.227ex" viewBox="0 -755.9 10582.1 958.9" role="img" focusable="false" style="vertical-align: -0.472ex;"><defs><path stroke-width="1" id="E128-MJMAIN-2212" 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600 417 472L151 346L399 228Q687 92 691 87Q694 81 694 76Q694 58 676 56H670L382 192Q92 329 90 331Q83 336 83 348Q84 359 96 365Q104 369 382 500T665 634Q669 636 674 636ZM94 170Q102 172 104 172Q110 171 254 103T535 -30T678 -98Q694 -106 694 -118Q694 -136 676 -138H670L382 -2Q92 135 90 137Q83 142 83 154Q84 164 94 170Z"></path><path stroke-width="1" id="E128-MJMATHI-78" d="M52 289Q59 331 106 386T222 442Q257 442 286 424T329 379Q371 442 430 442Q467 442 494 420T522 361Q522 332 508 314T481 292T458 288Q439 288 427 299T415 328Q415 374 465 391Q454 404 425 404Q412 404 406 402Q368 386 350 336Q290 115 290 78Q290 50 306 38T341 26Q378 26 414 59T463 140Q466 150 469 151T485 153H489Q504 153 504 145Q504 144 502 134Q486 77 440 33T333 -11Q263 -11 227 52Q186 -10 133 -10H127Q78 -10 57 16T35 71Q35 103 54 123T99 143Q142 143 142 101Q142 81 130 66T107 46T94 41L91 40Q91 39 97 36T113 29T132 26Q168 26 194 71Q203 87 217 139T245 247T261 313Q266 340 266 352Q266 380 251 392T217 404Q177 404 142 372T93 290Q91 281 88 280T72 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id="MathJax-Element-127">-0.00001 \leqslant x \leqslant 0.00001</script>，就显得过大/过小了。</p><p>另外注意，一旦采用特征缩放，我们就需对所有的输入采用特征缩放，包括训练集、测试集、预测输入等。</p><h2><a name='header-n82' class='md-header-anchor '></a>4.4 梯度下降实践2-学习速率(Gradient Descent in Practice II - Learning Rate)</h2><p>通常，有两种方法来确定函数是否收敛</p><ul><li><p>多次迭代收敛法</p><ul><li>无法确定需要多少次迭代</li><li>较易绘制关于迭代次数的图像</li><li>根据图像易预测所需的迭代次数</li></ul></li><li><p>自动化测试收敛法（比较阈值）</p><ul><li>不易选取阈值</li><li>损失函数近乎直线时无法确定收敛情况</li></ul></li></ul><p>对于梯度下降，一般采用多次迭代收敛法来得出最小化损失函数的参数值，自动化测试收敛法（如设定 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-128-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="12.514ex" height="2.928ex" viewBox="0 -956.9 5388.1 1260.5" role="img" focusable="false" style="vertical-align: -0.705ex;"><defs><path stroke-width="1" id="E129-MJMATHI-4A" d="M447 625Q447 637 354 637H329Q323 642 323 645T325 664Q329 677 335 683H352Q393 681 498 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y="0"></use><g transform="translate(1001,393)"><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E129-MJMAIN-2212" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E129-MJMAIN-33" x="778" y="0"></use></g></g></g></svg></span><script type="math/tex" id="MathJax-Element-128">J\left(\theta\right) < {10}^{-3}</script> 时判定收敛）则几乎不会被使用。</p><p>我们可以通过绘制<strong>损失函数关于迭代次数的图像</strong>，可视化梯度下降的执行过程，借助直观的图形来发现损失函数趋向于多少时能趋于收敛，依据图像变化情况，确定诸如学习速率的取值，迭代次数的大小等问题。</p><p><img src='image/20180108_103357.png' alt='' /></p><p>对于学习速率 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-145-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="1.488ex" height="1.41ex" viewBox="0 -504.6 640.5 607.1" role="img" focusable="false" style="vertical-align: -0.238ex;"><defs><path stroke-width="1" id="E146-MJMATHI-3B1" d="M34 156Q34 270 120 356T309 442Q379 442 421 402T478 304Q484 275 485 237V208Q534 282 560 374Q564 388 566 390T582 393Q603 393 603 385Q603 376 594 346T558 261T497 161L486 147L487 123Q489 67 495 47T514 26Q528 28 540 37T557 60Q559 67 562 68T577 70Q597 70 597 62Q597 56 591 43Q579 19 556 5T512 -10H505Q438 -10 414 62L411 69L400 61Q390 53 370 41T325 18T267 -2T203 -11Q124 -11 79 39T34 156ZM208 26Q257 26 306 47T379 90L403 112Q401 255 396 290Q382 405 304 405Q235 405 183 332Q156 292 139 224T121 120Q121 71 146 49T208 26Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E146-MJMATHI-3B1" x="0" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-145">\alpha</script>，一般上图展现的为适中情况，下图中，左图可能表明 <strong><span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-145-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="1.488ex" height="1.41ex" viewBox="0 -504.6 640.5 607.1" role="img" focusable="false" style="vertical-align: -0.238ex;"><defs><path stroke-width="1" id="E146-MJMATHI-3B1" d="M34 156Q34 270 120 356T309 442Q379 442 421 402T478 304Q484 275 485 237V208Q534 282 560 374Q564 388 566 390T582 393Q603 393 603 385Q603 376 594 346T558 261T497 161L486 147L487 123Q489 67 495 47T514 26Q528 28 540 37T557 60Q559 67 562 68T577 70Q597 70 597 62Q597 56 591 43Q579 19 556 5T512 -10H505Q438 -10 414 62L411 69L400 61Q390 53 370 41T325 18T267 -2T203 -11Q124 -11 79 39T34 156ZM208 26Q257 26 306 47T379 90L403 112Q401 255 396 290Q382 405 304 405Q235 405 183 332Q156 292 139 224T121 120Q121 71 146 49T208 26Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E146-MJMATHI-3B1" x="0" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-145">\alpha</script> 过大</strong>，损失函数<strong>无法收敛</strong>，右图可能表明 <strong><span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-145-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="1.488ex" height="1.41ex" viewBox="0 -504.6 640.5 607.1" role="img" focusable="false" style="vertical-align: -0.238ex;"><defs><path stroke-width="1" id="E146-MJMATHI-3B1" d="M34 156Q34 270 120 356T309 442Q379 442 421 402T478 304Q484 275 485 237V208Q534 282 560 374Q564 388 566 390T582 393Q603 393 603 385Q603 376 594 346T558 261T497 161L486 147L487 123Q489 67 495 47T514 26Q528 28 540 37T557 60Q559 67 562 68T577 70Q597 70 597 62Q597 56 591 43Q579 19 556 5T512 -10H505Q438 -10 414 62L411 69L400 61Q390 53 370 41T325 18T267 -2T203 -11Q124 -11 79 39T34 156ZM208 26Q257 26 306 47T379 90L403 112Q401 255 396 290Q382 405 304 405Q235 405 183 332Q156 292 139 224T121 120Q121 71 146 49T208 26Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E146-MJMATHI-3B1" x="0" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-145">\alpha</script> 过小</strong>，损失函数<strong>收敛的太慢</strong>。当然，<span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-145-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="1.488ex" height="1.41ex" viewBox="0 -504.6 640.5 607.1" role="img" focusable="false" style="vertical-align: -0.238ex;"><defs><path stroke-width="1" id="E146-MJMATHI-3B1" d="M34 156Q34 270 120 356T309 442Q379 442 421 402T478 304Q484 275 485 237V208Q534 282 560 374Q564 388 566 390T582 393Q603 393 603 385Q603 376 594 346T558 261T497 161L486 147L487 123Q489 67 495 47T514 26Q528 28 540 37T557 60Q559 67 562 68T577 70Q597 70 597 62Q597 56 591 43Q579 19 556 5T512 -10H505Q438 -10 414 62L411 69L400 61Q390 53 370 41T325 18T267 -2T203 -11Q124 -11 79 39T34 156ZM208 26Q257 26 306 47T379 90L403 112Q401 255 396 290Q382 405 304 405Q235 405 183 332Q156 292 139 224T121 120Q121 71 146 49T208 26Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E146-MJMATHI-3B1" x="0" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-145">\alpha</script> 足够小时，损失函数在每轮迭代后一定会减少。</p><p><img src='image/20180108_104701.png' alt='' /></p><p>通过不断改变 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-145-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="1.488ex" height="1.41ex" viewBox="0 -504.6 640.5 607.1" role="img" focusable="false" style="vertical-align: -0.238ex;"><defs><path stroke-width="1" id="E146-MJMATHI-3B1" d="M34 156Q34 270 120 356T309 442Q379 442 421 402T478 304Q484 275 485 237V208Q534 282 560 374Q564 388 566 390T582 393Q603 393 603 385Q603 376 594 346T558 261T497 161L486 147L487 123Q489 67 495 47T514 26Q528 28 540 37T557 60Q559 67 562 68T577 70Q597 70 597 62Q597 56 591 43Q579 19 556 5T512 -10H505Q438 -10 414 62L411 69L400 61Q390 53 370 41T325 18T267 -2T203 -11Q124 -11 79 39T34 156ZM208 26Q257 26 306 47T379 90L403 112Q401 255 396 290Q382 405 304 405Q235 405 183 332Q156 292 139 224T121 120Q121 71 146 49T208 26Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E146-MJMATHI-3B1" x="0" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-145">\alpha</script> 值，绘制并观察图像，并以此来确定合适的学习速率。 尝试时可取 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-145-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="1.488ex" height="1.41ex" viewBox="0 -504.6 640.5 607.1" role="img" focusable="false" style="vertical-align: -0.238ex;"><defs><path stroke-width="1" id="E146-MJMATHI-3B1" d="M34 156Q34 270 120 356T309 442Q379 442 421 402T478 304Q484 275 485 237V208Q534 282 560 374Q564 388 566 390T582 393Q603 393 603 385Q603 376 594 346T558 261T497 161L486 147L487 123Q489 67 495 47T514 26Q528 28 540 37T557 60Q559 67 562 68T577 70Q597 70 597 62Q597 56 591 43Q579 19 556 5T512 -10H505Q438 -10 414 62L411 69L400 61Q390 53 370 41T325 18T267 -2T203 -11Q124 -11 79 39T34 156ZM208 26Q257 26 306 47T379 90L403 112Q401 255 396 290Q382 405 304 405Q235 405 183 332Q156 292 139 224T121 120Q121 71 146 49T208 26Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E146-MJMATHI-3B1" x="0" y="0"></use></g></svg></span><script type="math/tex" 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Regression)</strong>。</p><p>比如一个二次方模型：<span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-136-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="25.85ex" height="3.044ex" viewBox="0 -906.7 11129.6 1310.7" role="img" focusable="false" style="vertical-align: -0.938ex;"><defs><path stroke-width="1" id="E137-MJMATHI-68" d="M137 683Q138 683 209 688T282 694Q294 694 294 685Q294 674 258 534Q220 386 220 383Q220 381 227 388Q288 442 357 442Q411 442 444 415T478 336Q478 285 440 178T402 50Q403 36 407 31T422 26Q450 26 474 56T513 138Q516 149 519 151T535 153Q555 153 555 145Q555 144 551 130Q535 71 500 33Q466 -10 419 -10H414Q367 -10 346 17T325 74Q325 90 361 192T398 345Q398 404 354 404H349Q266 404 205 306L198 293L164 158Q132 28 127 16Q114 -11 83 -11Q69 -11 59 -2T48 16Q48 30 121 320L195 616Q195 629 188 632T149 637H128Q122 643 122 645T124 664Q129 683 137 683Z"></path><path stroke-width="1" 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xlink:href="#E137-MJMAIN-29" x="962" y="0"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E137-MJMAIN-3D" x="2804" y="0"></use><g transform="translate(3860,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E137-MJMATHI-3B8" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E137-MJMAIN-30" x="663" y="-213"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E137-MJMAIN-2B" x="5006" y="0"></use><g transform="translate(6007,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E137-MJMATHI-3B8" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E137-MJMAIN-31" x="663" y="-213"></use></g><g transform="translate(6930,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E137-MJMATHI-78" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E137-MJMAIN-31" 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287Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E143-MJMATHI-3B8" x="0" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E143-MJMAIN-3D" x="747" y="0"></use><g transform="translate(1803,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E143-MJSZ1-28"></use><g transform="translate(458,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E143-MJMATHI-58" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E143-MJMATHI-54" x="1215" y="513"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E143-MJMATHI-58" x="1916" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E143-MJSZ1-29" x="2768" y="-1"></use><g transform="translate(3227,577)"><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E143-MJMAIN-2212" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E143-MJMAIN-31" x="778" y="0"></use></g></g><g transform="translate(6035,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E143-MJMATHI-58" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E143-MJMATHI-54" x="1215" y="513"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E143-MJMATHI-79" x="7493" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-142">\theta ={{\left( {X^T}X \right)}^{-1}}{X^{T}}y</script> ，Octave 中代码为 <code>theta = inv(X&#39;*X)*X&#39;*y</code>。</p><blockquote><p><span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-143-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="4.33ex" height="2.461ex" viewBox="0 -956.9 1864.1 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42T284 35Q284 26 282 19Q279 6 276 4T261 1Q258 1 243 1T201 2T142 2Q64 2 42 0Z"></path><path stroke-width="1" id="E144-MJMAIN-2212" d="M84 237T84 250T98 270H679Q694 262 694 250T679 230H98Q84 237 84 250Z"></path><path stroke-width="1" id="E144-MJMAIN-31" d="M213 578L200 573Q186 568 160 563T102 556H83V602H102Q149 604 189 617T245 641T273 663Q275 666 285 666Q294 666 302 660V361L303 61Q310 54 315 52T339 48T401 46H427V0H416Q395 3 257 3Q121 3 100 0H88V46H114Q136 46 152 46T177 47T193 50T201 52T207 57T213 61V578Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E144-MJMATHI-58" x="0" y="0"></use><g transform="translate(859,410)"><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E144-MJMAIN-2212" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E144-MJMAIN-31" x="778" y="0"></use></g></g></svg></span><script type="math/tex" id="MathJax-Element-143">{X}^{-1}</script>: 矩阵 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-144-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="1.98ex" height="1.994ex" viewBox="0 -755.9 852.5 858.4" role="img" focusable="false" style="vertical-align: -0.238ex;"><defs><path stroke-width="1" id="E145-MJMATHI-58" d="M42 0H40Q26 0 26 11Q26 15 29 27Q33 41 36 43T55 46Q141 49 190 98Q200 108 306 224T411 342Q302 620 297 625Q288 636 234 637H206Q200 643 200 645T202 664Q206 677 212 683H226Q260 681 347 681Q380 681 408 681T453 682T473 682Q490 682 490 671Q490 670 488 658Q484 643 481 640T465 637Q434 634 411 620L488 426L541 485Q646 598 646 610Q646 628 622 635Q617 635 609 637Q594 637 594 648Q594 650 596 664Q600 677 606 683H618Q619 683 643 683T697 681T738 680Q828 680 837 683H845Q852 676 852 672Q850 647 840 637H824Q790 636 763 628T722 611T698 593L687 584Q687 585 592 480L505 384Q505 383 536 304T601 142T638 56Q648 47 699 46Q734 46 734 37Q734 35 732 23Q728 7 725 4T711 1Q708 1 678 1T589 2Q528 2 496 2T461 1Q444 1 444 10Q444 11 446 25Q448 35 450 39T455 44T464 46T480 47T506 54Q523 62 523 64Q522 64 476 181L429 299Q241 95 236 84Q232 76 232 72Q232 53 261 47Q262 47 267 47T273 46Q276 46 277 46T280 45T283 42T284 35Q284 26 282 19Q279 6 276 4T261 1Q258 1 243 1T201 2T142 2Q64 2 42 0Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E145-MJMATHI-58" x="0" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-144">X</script> 的逆，在 Octave 中，<code>inv</code> 函数用于计算矩阵的逆，类似的还有 <code>pinv</code> 函数。</p></blockquote><p>下表列出了正规方程法与梯度下降算法的对比</p><figure><table><thead><tr><th>条件</th><th>梯度下降</th><th>正规方程</th></tr></thead><tbody><tr><td>是否需要选取 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-145-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="1.488ex" height="1.41ex" viewBox="0 -504.6 640.5 607.1" role="img" focusable="false" style="vertical-align: -0.238ex;"><defs><path stroke-width="1" id="E146-MJMATHI-3B1" d="M34 156Q34 270 120 356T309 442Q379 442 421 402T478 304Q484 275 485 237V208Q534 282 560 374Q564 388 566 390T582 393Q603 393 603 385Q603 376 594 346T558 261T497 161L486 147L487 123Q489 67 495 47T514 26Q528 28 540 37T557 60Q559 67 562 68T577 70Q597 70 597 62Q597 56 591 43Q579 19 556 5T512 -10H505Q438 -10 414 62L411 69L400 61Q390 53 370 41T325 18T267 -2T203 -11Q124 -11 79 39T34 156ZM208 26Q257 26 306 47T379 90L403 112Q401 255 396 290Q382 405 304 405Q235 405 183 332Q156 292 139 224T121 120Q121 71 146 49T208 26Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E146-MJMATHI-3B1" x="0" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-145">\alpha</script></td><td>需要</td><td>不需要</td></tr><tr><td>是否需要迭代运算</td><td>需要</td><td>不需要</td></tr><tr><td>特征量大<sup><a href='#dfref-footnote-1' name='ref-footnote-1'>1</a></sup>时</td><td>适用，<span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-146-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="7.95ex" height="3.044ex" viewBox="0 -906.7 3423.1 1310.7" role="img" focusable="false" style="vertical-align: -0.938ex;"><defs><path stroke-width="1" id="E147-MJMATHI-4F" d="M740 435Q740 320 676 213T511 42T304 -22Q207 -22 138 35T51 201Q50 209 50 244Q50 346 98 438T227 601Q351 704 476 704Q514 704 524 703Q621 689 680 617T740 435ZM637 476Q637 565 591 615T476 665Q396 665 322 605Q242 542 200 428T157 216Q157 126 200 73T314 19Q404 19 485 98T608 313Q637 408 637 476Z"></path><path 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x="3462" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E151-MJMATHI-3B8" x="4315" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E151-MJMAIN-2212" x="5006" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E151-MJMATHI-79" x="6007" y="0"></use><g transform="translate(6505,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E151-MJMAIN-29" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E151-MJMATHI-54" x="550" y="583"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E151-MJMAIN-28" x="7492" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E151-MJMATHI-58" x="7882" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E151-MJMATHI-3B8" x="8734" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E151-MJMAIN-2212" x="9426" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E151-MJMATHI-79" x="10427" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E151-MJMAIN-29" x="10924" y="0"></use></g></g></g></g></svg></span><script type="math/tex" id="MathJax-Element-150">\begin{aligned} & J\left( \theta  \right)=\frac{1}{2m}\sum\limits_{i=1}^{m}{{{\left( {h_{\theta}}\left( {x^{(i)}} \right)-{y^{(i)}} \right)}^{2}}}\newline \; & =\frac{1}{2m}||X\theta-y||^2 \newline \; & =\frac{1}{2m}(X\theta-y)^T(X\theta-y) &\newline  \end{aligned}</script></p><p>展开上式可得</p><p>​	<span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-151-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="46.505ex" height="3.395ex" viewBox="0 -1007.2 20022.7 1461.5" role="img" focusable="false" style="vertical-align: -1.055ex;"><defs><path stroke-width="1" id="E152-MJMATHI-4A" d="M447 625Q447 637 354 637H329Q323 642 323 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xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E152-MJMAIN-2B" x="12417" y="0"></use><g transform="translate(13417,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E152-MJMATHI-79" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E152-MJMATHI-54" x="706" y="513"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E152-MJMATHI-79" x="14515" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E152-MJSZ1-29" x="15013" y="-1"></use></g></g></svg></span><script type="math/tex" id="MathJax-Element-151">J(\theta )= \frac{1}{2m}\left( {{\theta }^{T}}{{X}^{T}}X\theta -{{\theta}^{T}}{{X}^{T}}y-{{y}^{T}}X\theta + {{y}^{T}}y \right)</script></p><p>注意到 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-152-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="7.021ex" 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y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E155-MJMAIN-2B" x="12578" y="0"></use><g transform="translate(13579,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E155-MJMATHI-79" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E155-MJMATHI-54" x="706" y="513"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E155-MJMATHI-79" x="14676" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E155-MJMAIN-5D" x="15174" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-154">J(\theta) = \frac{1}{2m}[X^TX\theta-2\theta^TX^Ty+y^Ty]</script></p><p>接下来对<span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-155-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="4.371ex" height="2.577ex" viewBox="0 -806.1 1882 1109.7" role="img" focusable="false" style="vertical-align: -0.705ex;"><defs><path stroke-width="1" id="E156-MJMATHI-4A" d="M447 625Q447 637 354 637H329Q323 642 323 645T325 664Q329 677 335 683H352Q393 681 498 681Q541 681 568 681T605 682T619 682Q633 682 633 672Q633 670 630 658Q626 642 623 640T604 637Q552 637 545 623Q541 610 483 376Q420 128 419 127Q397 64 333 21T195 -22Q137 -22 97 8T57 88Q57 130 80 152T132 174Q177 174 182 130Q182 98 164 80T123 56Q115 54 115 53T122 44Q148 15 197 15Q235 15 271 47T324 130Q328 142 387 380T447 625Z"></path><path stroke-width="1" id="E156-MJMAIN-28" d="M94 250Q94 319 104 381T127 488T164 576T202 643T244 695T277 729T302 750H315H319Q333 750 333 741Q333 738 316 720T275 667T226 581T184 443T167 250T184 58T225 -81T274 -167T316 -220T333 -241Q333 -250 318 -250H315H302L274 -226Q180 -141 137 -14T94 250Z"></path><path stroke-width="1" id="E156-MJMATHI-3B8" d="M35 200Q35 302 74 415T180 610T319 704Q320 704 327 704T339 705Q393 701 423 656Q462 596 462 495Q462 380 417 261T302 66T168 -10H161Q125 -10 99 10T60 63T41 130T35 200ZM383 566Q383 668 330 668Q294 668 260 623T204 521T170 421T157 371Q206 370 254 370L351 371Q352 372 359 404T375 484T383 566ZM113 132Q113 26 166 26Q181 26 198 36T239 74T287 161T335 307L340 324H145Q145 321 136 286T120 208T113 132Z"></path><path stroke-width="1" id="E156-MJMAIN-29" d="M60 749L64 750Q69 750 74 750H86L114 726Q208 641 251 514T294 250Q294 182 284 119T261 12T224 -76T186 -143T145 -194T113 -227T90 -246Q87 -249 86 -250H74Q66 -250 63 -250T58 -247T55 -238Q56 -237 66 -225Q221 -64 221 250T66 725Q56 737 55 738Q55 746 60 749Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E156-MJMATHI-4A" x="0" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E156-MJMAIN-28" x="633" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E156-MJMATHI-3B8" x="1023" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E156-MJMAIN-29" x="1492" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-155">J(\theta )</script> 求偏导，根据矩阵的求导法则:</p><p>​	<span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-156-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="21.356ex" height="3.745ex" viewBox="0 -1107.7 9195 1612.3" role="img" focusable="false" style="vertical-align: -1.172ex;"><defs><path stroke-width="1" id="E157-MJMATHI-64" d="M366 683Q367 683 438 688T511 694Q523 694 523 686Q523 679 450 384T375 83T374 68Q374 26 402 26Q411 27 422 35Q443 55 463 131Q469 151 473 152Q475 153 483 153H487H491Q506 153 506 145Q506 140 503 129Q490 79 473 48T445 8T417 -8Q409 -10 393 -10Q359 -10 336 5T306 36L300 51Q299 52 296 50Q294 48 292 46Q233 -10 172 -10Q117 -10 75 30T33 157Q33 205 53 255T101 341Q148 398 195 420T280 442Q336 442 364 400Q369 394 369 396Q370 400 396 505T424 616Q424 629 417 632T378 637H357Q351 643 351 645T353 664Q358 683 366 683ZM352 326Q329 405 277 405Q242 405 210 374T160 293Q131 214 119 129Q119 126 119 118T118 106Q118 61 136 44T179 26Q233 26 290 98L298 109L352 326Z"></path><path stroke-width="1" id="E157-MJMATHI-58" d="M42 0H40Q26 0 26 11Q26 15 29 27Q33 41 36 43T55 46Q141 49 190 98Q200 108 306 224T411 342Q302 620 297 625Q288 636 234 637H206Q200 643 200 645T202 664Q206 677 212 683H226Q260 681 347 681Q380 681 408 681T453 682T473 682Q490 682 490 671Q490 670 488 658Q484 643 481 640T465 637Q434 634 411 620L488 426L541 485Q646 598 646 610Q646 628 622 635Q617 635 609 637Q594 637 594 648Q594 650 596 664Q600 677 606 683H618Q619 683 643 683T697 681T738 680Q828 680 837 683H845Q852 676 852 672Q850 647 840 637H824Q790 636 763 628T722 611T698 593L687 584Q687 585 592 480L505 384Q505 383 536 304T601 142T638 56Q648 47 699 46Q734 46 734 37Q734 35 732 23Q728 7 725 4T711 1Q708 1 678 1T589 2Q528 2 496 2T461 1Q444 1 444 10Q444 11 446 25Q448 35 450 39T455 44T464 46T480 47T506 54Q523 62 523 64Q522 64 476 181L429 299Q241 95 236 84Q232 76 232 72Q232 53 261 47Q262 47 267 47T273 46Q276 46 277 46T280 45T283 42T284 35Q284 26 282 19Q279 6 276 4T261 1Q258 1 243 1T201 2T142 2Q64 2 42 0Z"></path><path stroke-width="1" id="E157-MJMATHI-54" d="M40 437Q21 437 21 445Q21 450 37 501T71 602L88 651Q93 669 101 677H569H659Q691 677 697 676T704 667Q704 661 687 553T668 444Q668 437 649 437Q640 437 637 437T631 442L629 445Q629 451 635 490T641 551Q641 586 628 604T573 629Q568 630 515 631Q469 631 457 630T439 622Q438 621 368 343T298 60Q298 48 386 46Q418 46 427 45T436 36Q436 31 433 22Q429 4 424 1L422 0Q419 0 415 0Q410 0 363 1T228 2Q99 2 64 0H49Q43 6 43 9T45 27Q49 40 55 46H83H94Q174 46 189 55Q190 56 191 56Q196 59 201 76T241 233Q258 301 269 344Q339 619 339 625Q339 630 310 630H279Q212 630 191 624Q146 614 121 583T67 467Q60 445 57 441T43 437H40Z"></path><path stroke-width="1" id="E157-MJMATHI-41" d="M208 74Q208 50 254 46Q272 46 272 35Q272 34 270 22Q267 8 264 4T251 0Q249 0 239 0T205 1T141 2Q70 2 50 0H42Q35 7 35 11Q37 38 48 46H62Q132 49 164 96Q170 102 345 401T523 704Q530 716 547 716H555H572Q578 707 578 706L606 383Q634 60 636 57Q641 46 701 46Q726 46 726 36Q726 34 723 22Q720 7 718 4T704 0Q701 0 690 0T651 1T578 2Q484 2 455 0H443Q437 6 437 9T439 27Q443 40 445 43L449 46H469Q523 49 533 63L521 213H283L249 155Q208 86 208 74ZM516 260Q516 271 504 416T490 562L463 519Q447 492 400 412L310 260L413 259Q516 259 516 260Z"></path><path stroke-width="1" id="E157-MJMAIN-3D" d="M56 347Q56 360 70 367H707Q722 359 722 347Q722 336 708 328L390 327H72Q56 332 56 347ZM56 153Q56 168 72 173H708Q722 163 722 153Q722 140 707 133H70Q56 140 56 153Z"></path><path stroke-width="1" id="E157-MJMAIN-28" d="M94 250Q94 319 104 381T127 488T164 576T202 643T244 695T277 729T302 750H315H319Q333 750 333 741Q333 738 316 720T275 667T226 581T184 443T167 250T184 58T225 -81T274 -167T316 -220T333 -241Q333 -250 318 -250H315H302L274 -226Q180 -141 137 -14T94 250Z"></path><path stroke-width="1" id="E157-MJMAIN-2B" d="M56 237T56 250T70 270H369V420L370 570Q380 583 389 583Q402 583 409 568V270H707Q722 262 722 250T707 230H409V-68Q401 -82 391 -82H389H387Q375 -82 369 -68V230H70Q56 237 56 250Z"></path><path stroke-width="1" id="E157-MJMAIN-54" d="M36 443Q37 448 46 558T55 671V677H666V671Q667 666 676 556T685 443V437H645V443Q645 445 642 478T631 544T610 593Q593 614 555 625Q534 630 478 630H451H443Q417 630 414 618Q413 616 413 339V63Q420 53 439 50T528 46H558V0H545L361 3Q186 1 177 0H164V46H194Q264 46 283 49T309 63V339V550Q309 620 304 625T271 630H244H224Q154 630 119 601Q101 585 93 554T81 486T76 443V437H36V443Z"></path><path stroke-width="1" id="E157-MJMAIN-29" d="M60 749L64 750Q69 750 74 750H86L114 726Q208 641 251 514T294 250Q294 182 284 119T261 12T224 -76T186 -143T145 -194T113 -227T90 -246Q87 -249 86 -250H74Q66 -250 63 -250T58 -247T55 -238Q56 -237 66 -225Q221 -64 221 250T66 725Q56 737 55 738Q55 746 60 749Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><g transform="translate(120,0)"><rect stroke="none" width="2654" height="60" x="0" y="220"></rect><g transform="translate(60,412)"><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E157-MJMATHI-64" x="0" y="0"></use><g transform="translate(370,0)"><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E157-MJMATHI-58" x="0" y="0"></use><use transform="scale(0.5)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E157-MJMATHI-54" x="1215" y="581"></use></g><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E157-MJMATHI-41" x="1981" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E157-MJMATHI-58" x="2731" y="0"></use></g><g transform="translate(840,-396)"><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E157-MJMATHI-64" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E157-MJMATHI-58" x="523" y="0"></use></g></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E157-MJMAIN-3D" x="3172" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E157-MJMAIN-28" x="4228" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E157-MJMATHI-41" x="4618" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E157-MJMAIN-2B" x="5590" y="0"></use><g transform="translate(6591,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E157-MJMATHI-41" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E157-MJMAIN-54" x="1061" y="513"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E157-MJMAIN-29" x="7952" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E157-MJMATHI-58" x="8342" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-156">\frac{dX^TAX}{dX}=(A+A^\mathrm{T})X</script></p><p>​	<span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-157-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="10.164ex" height="3.745ex" viewBox="0 -1107.7 4376.3 1612.3" role="img" focusable="false" style="vertical-align: -1.172ex;"><defs><path stroke-width="1" id="E158-MJMATHI-64" d="M366 683Q367 683 438 688T511 694Q523 694 523 686Q523 679 450 384T375 83T374 68Q374 26 402 26Q411 27 422 35Q443 55 463 131Q469 151 473 152Q475 153 483 153H487H491Q506 153 506 145Q506 140 503 129Q490 79 473 48T445 8T417 -8Q409 -10 393 -10Q359 -10 336 5T306 36L300 51Q299 52 296 50Q294 48 292 46Q233 -10 172 -10Q117 -10 75 30T33 157Q33 205 53 255T101 341Q148 398 195 420T280 442Q336 442 364 400Q369 394 369 396Q370 400 396 505T424 616Q424 629 417 632T378 637H357Q351 643 351 645T353 664Q358 683 366 683ZM352 326Q329 405 277 405Q242 405 210 374T160 293Q131 214 119 129Q119 126 119 118T118 106Q118 61 136 44T179 26Q233 26 290 98L298 109L352 326Z"></path><path stroke-width="1" id="E158-MJMATHI-58" d="M42 0H40Q26 0 26 11Q26 15 29 27Q33 41 36 43T55 46Q141 49 190 98Q200 108 306 224T411 342Q302 620 297 625Q288 636 234 637H206Q200 643 200 645T202 664Q206 677 212 683H226Q260 681 347 681Q380 681 408 681T453 682T473 682Q490 682 490 671Q490 670 488 658Q484 643 481 640T465 637Q434 634 411 620L488 426L541 485Q646 598 646 610Q646 628 622 635Q617 635 609 637Q594 637 594 648Q594 650 596 664Q600 677 606 683H618Q619 683 643 683T697 681T738 680Q828 680 837 683H845Q852 676 852 672Q850 647 840 637H824Q790 636 763 628T722 611T698 593L687 584Q687 585 592 480L505 384Q505 383 536 304T601 142T638 56Q648 47 699 46Q734 46 734 37Q734 35 732 23Q728 7 725 4T711 1Q708 1 678 1T589 2Q528 2 496 2T461 1Q444 1 444 10Q444 11 446 25Q448 35 450 39T455 44T464 46T480 47T506 54Q523 62 523 64Q522 64 476 181L429 299Q241 95 236 84Q232 76 232 72Q232 53 261 47Q262 47 267 47T273 46Q276 46 277 46T280 45T283 42T284 35Q284 26 282 19Q279 6 276 4T261 1Q258 1 243 1T201 2T142 2Q64 2 42 0Z"></path><path stroke-width="1" id="E158-MJMATHI-54" d="M40 437Q21 437 21 445Q21 450 37 501T71 602L88 651Q93 669 101 677H569H659Q691 677 697 676T704 667Q704 661 687 553T668 444Q668 437 649 437Q640 437 637 437T631 442L629 445Q629 451 635 490T641 551Q641 586 628 604T573 629Q568 630 515 631Q469 631 457 630T439 622Q438 621 368 343T298 60Q298 48 386 46Q418 46 427 45T436 36Q436 31 433 22Q429 4 424 1L422 0Q419 0 415 0Q410 0 363 1T228 2Q99 2 64 0H49Q43 6 43 9T45 27Q49 40 55 46H83H94Q174 46 189 55Q190 56 191 56Q196 59 201 76T241 233Q258 301 269 344Q339 619 339 625Q339 630 310 630H279Q212 630 191 624Q146 614 121 583T67 467Q60 445 57 441T43 437H40Z"></path><path stroke-width="1" id="E158-MJMATHI-41" d="M208 74Q208 50 254 46Q272 46 272 35Q272 34 270 22Q267 8 264 4T251 0Q249 0 239 0T205 1T141 2Q70 2 50 0H42Q35 7 35 11Q37 38 48 46H62Q132 49 164 96Q170 102 345 401T523 704Q530 716 547 716H555H572Q578 707 578 706L606 383Q634 60 636 57Q641 46 701 46Q726 46 726 36Q726 34 723 22Q720 7 718 4T704 0Q701 0 690 0T651 1T578 2Q484 2 455 0H443Q437 6 437 9T439 27Q443 40 445 43L449 46H469Q523 49 533 63L521 213H283L249 155Q208 86 208 74ZM516 260Q516 271 504 416T490 562L463 519Q447 492 400 412L310 260L413 259Q516 259 516 260Z"></path><path stroke-width="1" id="E158-MJMAIN-3D" d="M56 347Q56 360 70 367H707Q722 359 722 347Q722 336 708 328L390 327H72Q56 332 56 347ZM56 153Q56 168 72 173H708Q722 163 722 153Q722 140 707 133H70Q56 140 56 153Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><g transform="translate(120,0)"><rect stroke="none" width="2051" height="60" x="0" y="220"></rect><g transform="translate(60,412)"><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E158-MJMATHI-64" x="0" y="0"></use><g transform="translate(370,0)"><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E158-MJMATHI-58" x="0" y="0"></use><use transform="scale(0.5)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E158-MJMATHI-54" x="1215" y="581"></use></g><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E158-MJMATHI-41" x="1981" y="0"></use></g><g transform="translate(539,-396)"><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E158-MJMATHI-64" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E158-MJMATHI-58" x="523" y="0"></use></g></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E158-MJMAIN-3D" x="2569" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E158-MJMATHI-41" x="3625" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-157">\frac{dX^TA}{dX}={A}</script></p><p>所以有:</p><p>​	<span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-158-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="29.352ex" height="3.978ex" viewBox="0 -1208.2 12637.5 1712.8" role="img" focusable="false" style="vertical-align: -1.172ex;"><defs><path stroke-width="1" id="E159-MJMAIN-2202" d="M202 508Q179 508 169 520T158 547Q158 557 164 577T185 624T230 675T301 710L333 715H345Q378 715 384 714Q447 703 489 661T549 568T566 457Q566 362 519 240T402 53Q321 -22 223 -22Q123 -22 73 56Q42 102 42 148V159Q42 276 129 370T322 465Q383 465 414 434T455 367L458 378Q478 461 478 515Q478 603 437 639T344 676Q266 676 223 612Q264 606 264 572Q264 547 246 528T202 508ZM430 306Q430 372 401 400T333 428Q270 428 222 382Q197 354 183 323T150 221Q132 149 132 116Q132 21 232 21Q244 21 250 22Q327 35 374 112Q389 137 409 196T430 306Z"></path><path stroke-width="1" id="E159-MJMATHI-4A" d="M447 625Q447 637 354 637H329Q323 642 323 645T325 664Q329 677 335 683H352Q393 681 498 681Q541 681 568 681T605 682T619 682Q633 682 633 672Q633 670 630 658Q626 642 623 640T604 637Q552 637 545 623Q541 610 483 376Q420 128 419 127Q397 64 333 21T195 -22Q137 -22 97 8T57 88Q57 130 80 152T132 174Q177 174 182 130Q182 98 164 80T123 56Q115 54 115 53T122 44Q148 15 197 15Q235 15 271 47T324 130Q328 142 387 380T447 625Z"></path><path stroke-width="1" id="E159-MJMAIN-28" d="M94 250Q94 319 104 381T127 488T164 576T202 643T244 695T277 729T302 750H315H319Q333 750 333 741Q333 738 316 720T275 667T226 581T184 443T167 250T184 58T225 -81T274 -167T316 -220T333 -241Q333 -250 318 -250H315H302L274 -226Q180 -141 137 -14T94 250Z"></path><path stroke-width="1" id="E159-MJMATHI-3B8" d="M35 200Q35 302 74 415T180 610T319 704Q320 704 327 704T339 705Q393 701 423 656Q462 596 462 495Q462 380 417 261T302 66T168 -10H161Q125 -10 99 10T60 63T41 130T35 200ZM383 566Q383 668 330 668Q294 668 260 623T204 521T170 421T157 371Q206 370 254 370L351 371Q352 372 359 404T375 484T383 566ZM113 132Q113 26 166 26Q181 26 198 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={{\left( {X^{T}}X \right)}^{-1}}{X^{T}}y</script></div><h2><a name='header-n197' class='md-header-anchor '></a>4.7 不可逆性正规方程(Normal Equation Noninvertibility)</h2><p>（本部分内容为选讲）</p><p>正规方程无法应用于不可逆的矩阵，发生这种问题的概率很小，通常由于</p><ul><li><p>特征之间线性相关</p><p>比如同时包含英寸的尺寸和米为单位的尺寸两个特征，它们是线性相关的</p><p>即 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-161-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="17.059ex" height="3.044ex" viewBox="0 -1007.2 7344.7 1310.7" role="img" focusable="false" style="vertical-align: -0.705ex;"><defs><path stroke-width="1" id="E162-MJMATHI-78" d="M52 289Q59 331 106 386T222 442Q257 442 286 424T329 379Q371 442 430 442Q467 442 494 420T522 361Q522 332 508 314T481 292T458 288Q439 288 427 299T415 328Q415 374 465 391Q454 404 425 404Q412 404 406 402Q368 386 350 336Q290 115 290 78Q290 50 306 38T341 26Q378 26 414 59T463 140Q466 150 469 151T485 153H489Q504 153 504 145Q504 144 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415T503 336T465 179T427 52Q427 26 444 26Q450 26 453 27Q482 32 505 65T540 145Q542 153 560 153Q580 153 580 145Q580 144 576 130Q568 101 554 73T508 17T439 -10Q392 -10 371 17T350 73Q350 92 386 193T423 345Q423 404 379 404H374Q288 404 229 303L222 291L189 157Q156 26 151 16Q138 -11 108 -11Q95 -11 87 -5T76 7T74 17Q74 30 112 180T152 343Q153 348 153 366Q153 405 129 405Q91 405 66 305Q60 285 60 284Q58 278 41 278H27Q21 284 21 287Z"></path><path stroke-width="1" id="E163-MJMAIN-29" d="M60 749L64 750Q69 750 74 750H86L114 726Q208 641 251 514T294 250Q294 182 284 119T261 12T224 -76T186 -143T145 -194T113 -227T90 -246Q87 -249 86 -250H74Q66 -250 63 -250T58 -247T55 -238Q56 -237 66 -225Q221 -64 221 250T66 725Q56 737 55 738Q55 746 60 749Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E163-MJMAIN-28" x="0" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E163-MJMATHI-6D" x="389" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E163-MJAMS-2A7D" x="1545" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E163-MJMATHI-6E" x="2602" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E163-MJMAIN-29" x="3202" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-162">\left(m \leqslant n \right)</script>。</p></li></ul><p>如果发现 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-163-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="5.366ex" height="2.344ex" viewBox="0 -906.7 2310.4 1009.2" role="img" focusable="false" style="vertical-align: -0.238ex;"><defs><path stroke-width="1" id="E164-MJMATHI-58" d="M42 0H40Q26 0 26 11Q26 15 29 27Q33 41 36 43T55 46Q141 49 190 98Q200 108 306 224T411 342Q302 620 297 625Q288 636 234 637H206Q200 643 200 645T202 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437Q640 437 637 437T631 442L629 445Q629 451 635 490T641 551Q641 586 628 604T573 629Q568 630 515 631Q469 631 457 630T439 622Q438 621 368 343T298 60Q298 48 386 46Q418 46 427 45T436 36Q436 31 433 22Q429 4 424 1L422 0Q419 0 415 0Q410 0 363 1T228 2Q99 2 64 0H49Q43 6 43 9T45 27Q49 40 55 46H83H94Q174 46 189 55Q190 56 191 56Q196 59 201 76T241 233Q258 301 269 344Q339 619 339 625Q339 630 310 630H279Q212 630 191 624Q146 614 121 583T67 467Q60 445 57 441T43 437H40Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E164-MJMATHI-58" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E164-MJMATHI-54" x="1215" y="513"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E164-MJMATHI-58" x="1457" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-163">X^TX</script> 的结果不可逆，可尝试：</p><ul><li>减少多余/重复特征</li><li>增加训练集数量</li><li>使用正则化（后文）</li></ul><p>对于这类不可逆的矩阵，我们称之为<strong>奇异矩阵</strong>或<strong>退化矩阵</strong>。</p><p>这种情况下，如果还想使用正规方程法，在Octave中，可以选用 <code>pinv</code> 函数，<code>pinv</code> 区别于 <code>inv</code>，<code>pinv</code> 函数被称为伪逆函数，在矩阵不可逆的时候，使用这个函数仍可正确地计算出 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-65-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="1.09ex" height="2.11ex" viewBox="0 -806.1 469.5 908.7" role="img" focusable="false" style="vertical-align: -0.238ex;"><defs><path stroke-width="1" id="E66-MJMATHI-3B8" d="M35 200Q35 302 74 415T180 610T319 704Q320 704 327 704T339 705Q393 701 423 656Q462 596 462 495Q462 380 417 261T302 66T168 -10H161Q125 -10 99 10T60 63T41 130T35 200ZM383 566Q383 668 330 668Q294 668 260 623T204 521T170 421T157 371Q206 370 254 370L351 371Q352 372 359 404T375 484T383 566ZM113 132Q113 26 166 26Q181 26 198 36T239 74T287 161T335 307L340 324H145Q145 321 136 286T120 208T113 132Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E66-MJMATHI-3B8" x="0" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-65">\theta</script> 的值。</p><h1><a name='header-n229' class='md-header-anchor '></a>5 Octave/Matlab Tutorial</h1><p>复习时可直接倍速回顾视频，笔记整理暂留。</p><h2><a name='header-n232' class='md-header-anchor '></a>5.1 Basic Operations</h2><h2><a name='header-n233' class='md-header-anchor '></a>5.2 Moving Data Around</h2><h2><a name='header-n234' class='md-header-anchor '></a>5.3 Computing on Data</h2><h2><a name='header-n235' class='md-header-anchor '></a>5.4 Plotting Data</h2><h2><a name='header-n236' class='md-header-anchor '></a>5.5 Control Statements_ for, while, if statement</h2><h2><a name='header-n237' class='md-header-anchor '></a>5.6 Vectorization</h2><h2><a name='header-n238' class='md-header-anchor '></a>5.x 常用函数整理</h2><div class='footnotes-area'  ><hr/>
+<div class='footnote-line'><span class='md-fn-count'>1</span> 一般来说，当 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-149-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="1.395ex" height="1.438ex" viewBox="0 -505.2 600.5 619.1" role="img" focusable="false" style="vertical-align: -0.264ex;"><defs><path stroke-width="1" id="E150-MJMATHI-6E" d="M21 287Q22 293 24 303T36 341T56 388T89 425T135 442Q171 442 195 424T225 390T231 369Q231 367 232 367L243 378Q304 442 382 442Q436 442 469 415T503 336T465 179T427 52Q427 26 444 26Q450 26 453 27Q482 32 505 65T540 145Q542 153 560 153Q580 153 580 145Q580 144 576 130Q568 101 554 73T508 17T439 -10Q392 -10 371 17T350 73Q350 92 386 193T423 345Q423 404 379 404H374Q288 404 229 303L222 291L189 157Q156 26 151 16Q138 -11 108 -11Q95 -11 87 -5T76 7T74 17Q74 30 112 180T152 343Q153 348 153 366Q153 405 129 405Q91 405 66 305Q60 285 60 284Q58 278 41 278H27Q21 284 21 287Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E150-MJMATHI-6E" x="0" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-149">n</script> 超过 10000 时，对于正规方程而言，特征量较大。<a name='dfref-footnote-1' href='#ref-footnote-1' title='回到文档' class='reversefootnote' >↩</a></div>
 <div class='footnote-line'><span class='md-fn-count'>2</span> 梯度下降算法的普适性好，而对于特定的线性回归模型，正规方程是很好的替代品。<a name='dfref-footnote-2' href='#ref-footnote-2' title='回到文档' class='reversefootnote' >↩</a></div></div></div>
 </body>
 </html>
\ No newline at end of file
diff --git a/week2.md b/week2.md
index f62c81b..a0a575b 100644
--- a/week2.md
+++ b/week2.md
@@ -12,9 +12,9 @@
 
 > $n$: 特征的总数 
 >
->  ${x}^{\left( i \right)}$: 代表特征矩阵中第 $i$ 行，也就是第 $i$ 个训练实例。
+>  ${x}^{\left( i \right)}$: 代表样本矩阵中第 $i$ 行，也就是第 $i$ 个训练实例。
 >
->  ${x}_{j}^{\left( i \right)}$: 代表特征矩阵中第 $i$ 行的第 $j$ 个特征，也就是第 $i$ 个训练实例的第 $j$ 个特征。
+>  ${x}_{j}^{\left( i \right)}$: 代表样本矩阵中第 $i$ 行的第 $j$ 列，也就是第 $i$ 个训练实例的第 $j$ 个特征。
 
 参照上图，则有 ${x}^{(2)}\text{=}\begin{bmatrix} 1416\\\ 3\\\ 2\\\ 40 \end{bmatrix}, {x}^{(2)}_{1} = 1416$
 
@@ -22,14 +22,20 @@
 
 对于 $\theta_0$，和单特征中一样，我们将其看作基础数值。例如，房价的基础价格。
 
-参数向量的维度为 $n+1$，在特征向量中添加 $x_{0}$ 后，其维度也变为 $n+1$， 则运用线性代数，可对 $h$ 简化。 
+参数向量的维度为 $n+1$，在特征向量中添加 $x_{0}$ 后，其维度也变为 $n+1$， 则运用线性代数，可简化 $h$：
 
 $h_\theta\left(x\right)=\begin{bmatrix}\theta_0\; \theta_1\; ... \;\theta_n \end{bmatrix}\begin{bmatrix}x_0 \newline x_1 \newline \vdots \newline x_n\end{bmatrix}= \theta^T x$
 
 > $\theta^T$: $\theta$ 矩阵的转置
 >
+> $x$: 某个样本的特征向量，$n+1$ 维特征量向量
+>
 > $x_0$: 为了计算方便我们会假设 $x_0^{(i)} = 1$
 
+**注：该部分记号较多，记不住可随时回顾！**
+
+
+
 ## 4.2 多变量梯度下降(Gradient Descent for Multiple Variables)
 
 多变量损失函数类似于单变量损失函数，
@@ -38,11 +44,11 @@ $h_\theta\left(x\right)=\begin{bmatrix}\theta_0\; \theta_1\; ... \;\theta_n \end
 
 前文提到梯度下降对于最小化损失函数的通用性，则多变量梯度下降公式即
 
-$\begin{align*} & \text{repeat until convergence:} \; \lbrace \newline \; &{{\theta }_{j}}:={{\theta }_{j}}-\alpha \frac{\partial }{\partial {{\theta }_{j}}}J\left( {\theta_{0}},{\theta_{1}}...{\theta_{n}}  \right) \newline \rbrace \end{align*}​$
+$\begin{align*} & \text{repeat until convergence:} \; \lbrace \newline \; &{{\theta }_{j}}:={{\theta }_{j}}-\alpha \frac{\partial }{\partial {{\theta }_{j}}}J\left( {\theta_{0}},{\theta_{1}}...{\theta_{n}}  \right) \newline \rbrace \end{align*}$
 
-对其求导：
+解出偏导得：
 
-$\begin{align*}& \text{repeat until convergence:} \; \lbrace \newline \; & \theta_j := \theta_j - \alpha \frac{1}{m} \sum\limits_{i=1}^{m} (h_\theta(x^{(i)}) - y^{(i)}) \cdot x_j^{(i)} \; & \text{for j := 0,1...n}\newline \rbrace\end{align*}$
+$\begin{align*}& \text{repeat until convergence:} \; \lbrace \newline \; & \theta_j := \theta_j - \alpha \frac{1}{m} \sum\limits_{i=1}^{m} (h_\theta(x^{(i)}) - y^{(i)}) \cdot x_j^{(i)} \; & \text{for j := 0,1...n}\newline \rbrace\end{align*}​$
 
 可展开为：
 
@@ -55,6 +61,8 @@ $$
 \theta = \theta - \alpha \frac{1}{m}(X^T(X\theta-y))
 $$
 
+> $X$: 训练集数据，$m\times(n+1)$ 维矩阵（包含额外特征 $x_0=1$）
+
 ## 4.3 梯度下降实践1-特征值缩放(Gradient Descent in Practice I - Feature Scaling)
 
 在应用梯度下降算法实践时，由于各特征值的范围不一，可能会影响损失函数收敛速度。
@@ -150,16 +158,12 @@ $$
 
 ​	$J(\theta) = \frac{1}{2m}[X^TX\theta-2\theta^TX^Ty+y^Ty]$
 
-
-
 接下来对$J(\theta )$ 求偏导，根据矩阵的求导法则:
 
 ​	$\frac{dX^TAX}{dX}=(A+A^\mathrm{T})X$
 
 ​	$\frac{dX^TA}{dX}={A}$
 
-
-
 所以有:
 
 ​	$\frac{\partial J\left( \theta  \right)}{\partial \theta }=\frac{1}{2m}\left(2{{X}^{T}}X\theta -2{{X}^{T}}y \right)$
diff --git a/week3.html b/week3.html
index bb99415..152c67d 100644
--- a/week3.html
+++ b/week3.html
@@ -220,6 +220,15 @@ header, .context-menu, .megamenu-content, footer { font-family: "Segoe UI", Aria
 </style>
 </head>
 <body class='typora-export' >
-<div  id='write'  class = 'is-node show-fences-line-number'><div class='md-toc' mdtype='toc'><p class="md-toc-content"><span class="md-toc-item md-toc-h1" data-ref="n216"><a class="md-toc-inner" style="cursor: pointer;" href="#header-n216">6 逻辑回归(Logistic Regression)</a></span><span class="md-toc-item md-toc-h2" data-ref="n217"><a class="md-toc-inner" style="cursor: pointer;" href="#header-n217">6.1 分类(Classification)</a></span><span class="md-toc-item md-toc-h2" data-ref="n254"><a class="md-toc-inner" style="cursor: pointer;" href="#header-n254">6.2 假设函数表示(Hypothesis Representation)</a></span><span class="md-toc-item md-toc-h2" data-ref="n278"><a class="md-toc-inner" style="cursor: pointer;" href="#header-n278">6.3 决策边界(Decision Boundary)</a></span><span class="md-toc-item md-toc-h2" data-ref="n291"><a class="md-toc-inner" style="cursor: pointer;" href="#header-n291">6.4 代价函数(Cost Function)</a></span><span class="md-toc-item md-toc-h2" data-ref="n292"><a class="md-toc-inner" style="cursor: pointer;" href="#header-n292">6.5 Simplified Cost Function and Gradient Descent</a></span><span class="md-toc-item md-toc-h2" data-ref="n293"><a class="md-toc-inner" style="cursor: pointer;" href="#header-n293">6.6 Advanced Optimization</a></span><span class="md-toc-item md-toc-h2" data-ref="n294"><a class="md-toc-inner" style="cursor: pointer;" href="#header-n294">6.7 Multiclass Classification_ One-vs-all</a></span><span class="md-toc-item md-toc-h1" data-ref="n295"><a class="md-toc-inner" style="cursor: pointer;" href="#header-n295">7 Regularization</a></span><span class="md-toc-item md-toc-h2" data-ref="n296"><a class="md-toc-inner" style="cursor: pointer;" href="#header-n296">7.1 The Problem of Overfitting</a></span><span class="md-toc-item md-toc-h2" data-ref="n297"><a class="md-toc-inner" style="cursor: pointer;" href="#header-n297">7.2 Cost Function</a></span><span class="md-toc-item md-toc-h2" data-ref="n298"><a class="md-toc-inner" style="cursor: pointer;" href="#header-n298">7.3 Regularized Linear Regression</a></span><span class="md-toc-item md-toc-h2" data-ref="n299"><a class="md-toc-inner" style="cursor: pointer;" href="#header-n299">7.4 Regularized Logistic Regression</a></span></p></div><h1><a name='header-n216' class='md-header-anchor '></a>6 逻辑回归(Logistic Regression)</h1><h2><a name='header-n217' class='md-header-anchor '></a>6.1 分类(Classification)</h2><p>在分类问题中，预测的结果是离散值（结果是否属于某一类），逻辑回归算法(Logistic Regression)被用于解决这类分类问题。</p><ul><li>垃圾邮件判断</li><li>金融欺诈判断</li><li>肿瘤诊断</li></ul><p>肿瘤诊断问题：</p><p><img src='image/20180109_144040.png' alt='' /></p><p>肿瘤诊断问题是一个<strong>二元分类问题(binary class problems)</strong>，则定义 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-2-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="9.68ex" height="2.577ex" viewBox="0 -806.1 4167.7 1109.7" role="img" focusable="false" style="vertical-align: 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xlink:href="#E4-MJMATHI-3B8" x="815" y="-219"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E4-MJMAIN-28" x="1008" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E4-MJMATHI-78" x="1397" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E4-MJMAIN-29" x="1970" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E4-MJAMS-2A7E" x="2637" y="0"></use><g transform="translate(3694,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E4-MJMAIN-30"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E4-MJMAIN-2E" x="500" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E4-MJMAIN-35" x="779" y="0"></use></g></g></svg></span><script type="math/tex" id="MathJax-Element-3">h_\theta(x) \geqslant 0.5</script> ，预测为 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-4-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="5.416ex" height="2.461ex" viewBox="0 -755.9 2332.1 1059.4" role="img" focusable="false" style="vertical-align: -0.705ex;"><defs><path stroke-width="1" id="E5-MJMATHI-79" d="M21 287Q21 301 36 335T84 406T158 442Q199 442 224 419T250 355Q248 336 247 334Q247 331 231 288T198 191T182 105Q182 62 196 45T238 27Q261 27 281 38T312 61T339 94Q339 95 344 114T358 173T377 247Q415 397 419 404Q432 431 462 431Q475 431 483 424T494 412T496 403Q496 390 447 193T391 -23Q363 -106 294 -155T156 -205Q111 -205 77 -183T43 -117Q43 -95 50 -80T69 -58T89 -48T106 -45Q150 -45 150 -87Q150 -107 138 -122T115 -142T102 -147L99 -148Q101 -153 118 -160T152 -167H160Q177 -167 186 -165Q219 -156 247 -127T290 -65T313 -9T321 21L315 17Q309 13 296 6T270 -6Q250 -11 231 -11Q185 -11 150 11T104 82Q103 89 103 113Q103 170 138 262T173 379Q173 380 173 381Q173 390 173 393T169 400T158 404H154Q131 404 112 385T82 344T65 302T57 280Q55 278 41 278H27Q21 284 21 287Z"></path><path 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xlink:href="#E6-MJMAIN-35" x="779" y="0"></use></g></g></svg></span><script type="math/tex" id="MathJax-Element-5">h_\theta(x) \lt 0.5</script> ，预测为 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-6-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="5.416ex" height="2.461ex" viewBox="0 -755.9 2332.1 1059.4" role="img" focusable="false" style="vertical-align: -0.705ex;"><defs><path stroke-width="1" id="E7-MJMATHI-79" d="M21 287Q21 301 36 335T84 406T158 442Q199 442 224 419T250 355Q248 336 247 334Q247 331 231 288T198 191T182 105Q182 62 196 45T238 27Q261 27 281 38T312 61T339 94Q339 95 344 114T358 173T377 247Q415 397 419 404Q432 431 462 431Q475 431 483 424T494 412T496 403Q496 390 447 193T391 -23Q363 -106 294 -155T156 -205Q111 -205 77 -183T43 -117Q43 -95 50 -80T69 -58T89 -48T106 -45Q150 -45 150 -87Q150 -107 138 -122T115 -142T102 -147L99 -148Q101 -153 118 -160T152 -167H160Q177 -167 186 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xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E7-MJMATHI-79" x="0" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E7-MJMAIN-3D" x="775" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E7-MJMAIN-30" x="1831" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-6">y = 0</script>，即负向类。</p><p>即以 0.5 为<strong>阈值</strong>(threshold)，则我们就可以根据线性回归结果，得到相对正确的分类结果 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-7-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="1.155ex" height="1.877ex" viewBox="0 -504.6 497.5 808.1" role="img" focusable="false" style="vertical-align: -0.705ex;"><defs><path stroke-width="1" id="E8-MJMATHI-79" d="M21 287Q21 301 36 335T84 406T158 442Q199 442 224 419T250 355Q248 336 247 334Q247 331 231 288T198 191T182 105Q182 62 196 45T238 27Q261 27 281 38T312 61T339 94Q339 95 344 114T358 173T377 247Q415 397 419 404Q432 431 462 431Q475 431 483 424T494 412T496 403Q496 390 447 193T391 -23Q363 -106 294 -155T156 -205Q111 -205 77 -183T43 -117Q43 -95 50 -80T69 -58T89 -48T106 -45Q150 -45 150 -87Q150 -107 138 -122T115 -142T102 -147L99 -148Q101 -153 118 -160T152 -167H160Q177 -167 186 -165Q219 -156 247 -127T290 -65T313 -9T321 21L315 17Q309 13 296 6T270 -6Q250 -11 231 -11Q185 -11 150 11T104 82Q103 89 103 113Q103 170 138 262T173 379Q173 380 173 381Q173 390 173 393T169 400T158 404H154Q131 404 112 385T82 344T65 302T57 280Q55 278 41 278H27Q21 284 21 287Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E8-MJMATHI-79" x="0" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-7">y</script>。</p><p>&nbsp;</p><p>接下来加入偏差项，线性回归算法给出了靛青色的拟合直线，如果阈值仍然为 0.5，可以看到算法在某些情况下会给出完全错误的结果。</p><p>不仅如此，线性回归算法的值域为 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-8-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="1.764ex" height="1.994ex" viewBox="0 -755.9 759.5 858.4" role="img" focusable="false" style="vertical-align: -0.238ex;"><defs><path stroke-width="1" id="E9-MJMATHI-52" d="M230 637Q203 637 198 638T193 649Q193 676 204 682Q206 683 378 683Q550 682 564 680Q620 672 658 652T712 606T733 563T739 529Q739 484 710 445T643 385T576 351T538 338L545 333Q612 295 612 223Q612 212 607 162T602 80V71Q602 53 603 43T614 25T640 16Q668 16 686 38T712 85Q717 99 720 102T735 105Q755 105 755 93Q755 75 731 36Q693 -21 641 -21H632Q571 -21 531 4T487 82Q487 109 502 166T517 239Q517 290 474 313Q459 320 449 321T378 323H309L277 193Q244 61 244 59Q244 55 245 54T252 50T269 48T302 46H333Q339 38 339 37T336 19Q332 6 326 0H311Q275 2 180 2Q146 2 117 2T71 2T50 1Q33 1 33 10Q33 12 36 24Q41 43 46 45Q50 46 61 46H67Q94 46 127 49Q141 52 146 61Q149 65 218 339T287 628Q287 635 230 637ZM630 554Q630 586 609 608T523 636Q521 636 500 636T462 637H440Q393 637 386 627Q385 624 352 494T319 361Q319 360 388 360Q466 361 492 367Q556 377 592 426Q608 449 619 486T630 554Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E9-MJMATHI-52" x="0" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-8">R</script>，则当线性回归函数给出诸如 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-9-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="23.342ex" height="2.344ex" viewBox="0 -755.9 10049.8 1009.2" role="img" focusable="false" style="vertical-align: -0.588ex;"><defs><path stroke-width="1" id="E10-MJMATHI-68" d="M137 683Q138 683 209 688T282 694Q294 694 294 685Q294 674 258 534Q220 386 220 383Q220 381 227 388Q288 442 357 442Q411 442 444 415T478 336Q478 285 440 178T402 50Q403 36 407 31T422 26Q450 26 474 56T513 138Q516 149 519 151T535 153Q555 153 555 145Q555 144 551 130Q535 71 500 33Q466 -10 419 -10H414Q367 -10 346 17T325 74Q325 90 361 192T398 345Q398 404 354 404H349Q266 404 205 306L198 293L164 158Q132 28 127 16Q114 -11 83 -11Q69 -11 59 -2T48 16Q48 30 121 320L195 616Q195 629 188 632T149 637H128Q122 643 122 645T124 664Q129 683 137 683Z"></path><path stroke-width="1" id="E10-MJMAIN-3D" d="M56 347Q56 360 70 367H707Q722 359 722 347Q722 336 708 328L390 327H72Q56 332 56 347ZM56 153Q56 168 72 173H708Q722 163 722 153Q722 140 707 133H70Q56 140 56 153Z"></path><path stroke-width="1" id="E10-MJMAIN-31" d="M213 578L200 573Q186 568 160 563T102 556H83V602H102Q149 604 189 617T245 641T273 663Q275 666 285 666Q294 666 302 660V361L303 61Q310 54 315 52T339 48T401 46H427V0H416Q395 3 257 3Q121 3 100 0H88V46H114Q136 46 152 46T177 47T193 50T201 52T207 57T213 61V578Z"></path><path stroke-width="1" id="E10-MJMAIN-30" d="M96 585Q152 666 249 666Q297 666 345 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transform="translate(1910,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E10-MJMAIN-31"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E10-MJMAIN-30" x="500" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E10-MJMAIN-30" x="1001" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E10-MJMAIN-30" x="1501" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E10-MJMAIN-30" x="2002" y="0"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E10-MJMAIN-2C" x="4413" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E10-MJMATHI-68" x="4858" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E10-MJMAIN-3D" x="5712" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E10-MJMAIN-2212" x="6768" y="0"></use><g transform="translate(7547,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E10-MJMAIN-31"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E10-MJMAIN-30" x="500" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E10-MJMAIN-30" x="1001" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E10-MJMAIN-30" x="1501" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E10-MJMAIN-30" x="2002" y="0"></use></g></g></svg></span><script type="math/tex" id="MathJax-Element-9">h = 10000, h = -10000</script> 等很大/很小(负数)的数值时，结果 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-10-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="9.68ex" height="2.577ex" viewBox="0 -806.1 4167.7 1109.7" role="img" focusable="false" style="vertical-align: -0.705ex;"><defs><path stroke-width="1" id="E11-MJMATHI-79" d="M21 287Q21 301 36 335T84 406T158 442Q199 442 224 419T250 355Q248 336 247 334Q247 331 231 288T198 191T182 105Q182 62 196 45T238 27Q261 27 281 38T312 61T339 94Q339 95 344 114T358 173T377 247Q415 397 419 404Q432 431 462 431Q475 431 483 424T494 412T496 403Q496 390 447 193T391 -23Q363 -106 294 -155T156 -205Q111 -205 77 -183T43 -117Q43 -95 50 -80T69 -58T89 -48T106 -45Q150 -45 150 -87Q150 -107 138 -122T115 -142T102 -147L99 -148Q101 -153 118 -160T152 -167H160Q177 -167 186 -165Q219 -156 247 -127T290 -65T313 -9T321 21L315 17Q309 13 296 6T270 -6Q250 -11 231 -11Q185 -11 150 11T104 82Q103 89 103 113Q103 170 138 262T173 379Q173 380 173 381Q173 390 173 393T169 400T158 404H154Q131 404 112 385T82 344T65 302T57 280Q55 278 41 278H27Q21 284 21 287Z"></path><path stroke-width="1" id="E11-MJMAIN-2208" d="M84 250Q84 372 166 450T360 539Q361 539 377 539T419 540T469 540H568Q583 532 583 520Q583 511 570 501L466 500Q355 499 329 494Q280 482 242 458T183 409T147 354T129 306T124 272V270H568Q583 262 583 250T568 230H124V228Q124 207 134 177T167 112T231 48T328 7Q355 1 466 0H570Q583 -10 583 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597Q291 629 250 629Q208 629 178 597Q153 571 145 525T137 333Q137 175 145 125T181 46Q209 16 250 16Q290 16 318 46Q347 76 354 130T362 333Q362 478 354 524T321 597Z"></path><path stroke-width="1" id="E11-MJMAIN-2C" d="M78 35T78 60T94 103T137 121Q165 121 187 96T210 8Q210 -27 201 -60T180 -117T154 -158T130 -185T117 -194Q113 -194 104 -185T95 -172Q95 -168 106 -156T131 -126T157 -76T173 -3V9L172 8Q170 7 167 6T161 3T152 1T140 0Q113 0 96 17Z"></path><path stroke-width="1" id="E11-MJMAIN-31" d="M213 578L200 573Q186 568 160 563T102 556H83V602H102Q149 604 189 617T245 641T273 663Q275 666 285 666Q294 666 302 660V361L303 61Q310 54 315 52T339 48T401 46H427V0H416Q395 3 257 3Q121 3 100 0H88V46H114Q136 46 152 46T177 47T193 50T201 52T207 57T213 61V578Z"></path><path stroke-width="1" id="E11-MJMAIN-7D" d="M65 731Q65 745 68 747T88 750Q171 750 216 725T279 670Q288 649 289 635T291 501Q292 362 293 357Q306 312 345 291T417 269Q428 269 431 266T434 250T431 234T417 231Q380 231 345 210T298 157Q293 143 292 121T291 -28V-79Q291 -134 285 -156T256 -198Q202 -250 89 -250Q71 -250 68 -247T65 -230Q65 -224 65 -223T66 -218T69 -214T77 -213Q91 -213 108 -210T146 -200T183 -177T207 -139Q208 -134 209 3L210 139Q223 196 280 230Q315 247 330 250Q305 257 280 270Q225 304 212 352L210 362L209 498Q208 635 207 640Q195 680 154 696T77 713Q68 713 67 716T65 731Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E11-MJMATHI-79" x="0" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E11-MJMAIN-2208" x="775" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E11-MJMAIN-7B" x="1720" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E11-MJMAIN-30" x="2221" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E11-MJMAIN-2C" x="2721" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E11-MJMAIN-31" x="3166" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E11-MJMAIN-7D" x="3667" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-10">y \in \lbrace 0, 1\rbrace</script>，这显得非常怪异。</p><p>&nbsp;</p><p>区别于线性回归算法，逻辑回归算法是一个分类算法，<strong>其输出值永远在 0 到 1 之间</strong>，即 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-11-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="9.348ex" height="2.577ex" viewBox="0 -806.1 4024.7 1109.7" role="img" focusable="false" style="vertical-align: -0.705ex;"><defs><path stroke-width="1" id="E12-MJMATHI-68" d="M137 683Q138 683 209 688T282 694Q294 694 294 685Q294 674 258 534Q220 386 220 383Q220 381 227 388Q288 442 357 442Q411 442 444 415T478 336Q478 285 440 178T402 50Q403 36 407 31T422 26Q450 26 474 56T513 138Q516 149 519 151T535 153Q555 153 555 145Q555 144 551 130Q535 71 500 33Q466 -10 419 -10H414Q367 -10 346 17T325 74Q325 90 361 192T398 345Q398 404 354 404H349Q266 404 205 306L198 293L164 158Q132 28 127 16Q114 -11 83 -11Q69 -11 59 -2T48 16Q48 30 121 320L195 616Q195 629 188 632T149 637H128Q122 643 122 645T124 664Q129 683 137 683Z"></path><path stroke-width="1" id="E12-MJMAIN-2208" d="M84 250Q84 372 166 450T360 539Q361 539 377 539T419 540T469 540H568Q583 532 583 520Q583 511 570 501L466 500Q355 499 329 494Q280 482 242 458T183 409T147 354T129 306T124 272V270H568Q583 262 583 250T568 230H124V228Q124 207 134 177T167 112T231 48T328 7Q355 1 466 0H570Q583 -10 583 -20Q583 -32 568 -40H471Q464 -40 446 -40T417 -41Q262 -41 172 45Q84 127 84 250Z"></path><path stroke-width="1" id="E12-MJMAIN-28" d="M94 250Q94 319 104 381T127 488T164 576T202 643T244 695T277 729T302 750H315H319Q333 750 333 741Q333 738 316 720T275 667T226 581T184 443T167 250T184 58T225 -81T274 -167T316 -220T333 -241Q333 -250 318 -250H315H302L274 -226Q180 -141 137 -14T94 250Z"></path><path stroke-width="1" id="E12-MJMAIN-30" d="M96 585Q152 666 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y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-11">h \in (0,1)</script>。</p><h2><a name='header-n254' class='md-header-anchor '></a>6.2 假设函数表示(Hypothesis Representation)</h2><p>为了使 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-12-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="9.348ex" height="2.577ex" viewBox="0 -806.1 4024.7 1109.7" role="img" focusable="false" style="vertical-align: -0.705ex;"><defs><path stroke-width="1" id="E13-MJMATHI-68" d="M137 683Q138 683 209 688T282 694Q294 694 294 685Q294 674 258 534Q220 386 220 383Q220 381 227 388Q288 442 357 442Q411 442 444 415T478 336Q478 285 440 178T402 50Q403 36 407 31T422 26Q450 26 474 56T513 138Q516 149 519 151T535 153Q555 153 555 145Q555 144 551 130Q535 71 500 33Q466 -10 419 -10H414Q367 -10 346 17T325 74Q325 90 361 192T398 345Q398 404 354 404H349Q266 404 205 306L198 293L164 158Q132 28 127 16Q114 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transform="translate(3860,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E14-MJMATHI-3B8" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E14-MJMATHI-54" x="663" y="513"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E14-MJMATHI-58" x="4928" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-13">h_\theta \left( x \right)=\theta^{T}X</script>，<span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-14-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="1.116ex" height="1.877ex" viewBox="0 -504.6 480.5 808.1" role="img" focusable="false" style="vertical-align: -0.705ex;"><defs><path stroke-width="1" id="E15-MJMATHI-67" d="M311 43Q296 30 267 15T206 0Q143 0 105 45T66 160Q66 265 143 353T314 442Q361 442 401 394L404 398Q406 401 409 404T418 412T431 419T447 422Q461 422 470 413T480 394Q480 379 423 152T363 -80Q345 -134 286 -169T151 -205Q10 -205 10 -137Q10 -111 28 -91T74 -71Q89 -71 102 -80T116 -111Q116 -121 114 -130T107 -144T99 -154T92 -162L90 -164H91Q101 -167 151 -167Q189 -167 211 -155Q234 -144 254 -122T282 -75Q288 -56 298 -13Q311 35 311 43ZM384 328L380 339Q377 350 375 354T369 368T359 382T346 393T328 402T306 405Q262 405 221 352Q191 313 171 233T151 117Q151 38 213 38Q269 38 323 108L331 118L384 328Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E15-MJMATHI-67" x="0" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-14">g</script> 表示逻辑函数(<a href='https://en.wikipedia.org/wiki/Logistic_function' target='_blank'>logistic function</a>)，复合起来，则称为逻辑回归函数。</p><p>逻辑函数是 S 形函数，会将所有实数映射到 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-15-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="5.168ex" height="2.577ex" viewBox="0 -806.1 2225.2 1109.7" role="img" focusable="false" style="vertical-align: -0.705ex;"><defs><path stroke-width="1" id="E16-MJMAIN-28" d="M94 250Q94 319 104 381T127 488T164 576T202 643T244 695T277 729T302 750H315H319Q333 750 333 741Q333 738 316 720T275 667T226 581T184 443T167 250T184 58T225 -81T274 -167T316 -220T333 -241Q333 -250 318 -250H315H302L274 -226Q180 -141 137 -14T94 250Z"></path><path stroke-width="1" id="E16-MJMAIN-30" d="M96 585Q152 666 249 666Q297 666 345 640T423 548Q460 465 460 320Q460 165 417 83Q397 41 362 16T301 -15T250 -22Q224 -22 198 -16T137 16T82 83Q39 165 39 320Q39 494 96 585ZM321 597Q291 629 250 629Q208 629 178 597Q153 571 145 525T137 333Q137 175 145 125T181 46Q209 16 250 16Q290 16 318 46Q347 76 354 130T362 333Q362 478 354 524T321 597Z"></path><path stroke-width="1" id="E16-MJMAIN-2C" d="M78 35T78 60T94 103T137 121Q165 121 187 96T210 8Q210 -27 201 -60T180 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xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E16-MJMAIN-30" x="389" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E16-MJMAIN-2C" x="890" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E16-MJMAIN-31" x="1335" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E16-MJMAIN-29" x="1835" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-15">(0, 1)</script> 范围。</p><p><a href='https://en.wikipedia.org/wiki/Sigmoid_function' target='_blank'>sigmoid 函数</a>（如下图）是逻辑函数的特殊情况，其公式为 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-16-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="12.814ex" height="3.395ex" viewBox="0 -956.9 5517.2 1461.5" role="img" focusable="false" style="vertical-align: -1.172ex;"><defs><path stroke-width="1" id="E17-MJMATHI-67" d="M311 43Q296 30 267 15T206 0Q143 0 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y="572"></use><g transform="translate(59,-376)"><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E17-MJMAIN-31" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E17-MJMAIN-2B" x="500" y="0"></use><g transform="translate(904,0)"><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E17-MJMATHI-65" x="0" y="0"></use><g transform="translate(329,204)"><use transform="scale(0.5)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E17-MJMAIN-2212" x="0" y="0"></use><use transform="scale(0.5)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E17-MJMATHI-7A" x="778" y="0"></use></g></g></g></g></g></g></svg></span><script type="math/tex" id="MathJax-Element-16">g\left( z \right)=\frac{1}{1+{{e}^{-z}}}</script>。 </p><p><img src='image/2413fbec8ff9fa1f19aaf78265b8a33b_Logistic_function.png' alt='sigmoid function' /></p><p>应用 sigmoid 函数，则逻辑回归模型：<span 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transform="translate(60,-608)"><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E18-MJMAIN-31" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E18-MJMAIN-2B" x="500" y="0"></use><g transform="translate(904,0)"><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E18-MJMATHI-65" x="0" y="0"></use><g transform="translate(329,204)"><use transform="scale(0.5)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E18-MJMAIN-2212" x="0" y="0"></use><g transform="translate(389,0)"><use transform="scale(0.5)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E18-MJMATHI-3B8" x="0" y="0"></use><use transform="scale(0.5)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E18-MJMATHI-54" x="469" y="319"></use></g><use transform="scale(0.5)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E18-MJMATHI-78" x="2052" y="0"></use></g></g></g></g></g></g></svg></span><script type="math/tex" id="MathJax-Element-17">h_{\theta}(x)=g(\theta^Tx) =\frac{1}{1+e^{-\theta^Tx}}</script></p><p>逻辑回归模型中，<span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-18-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="5.868ex" height="2.577ex" viewBox="0 -806.1 2526.7 1109.7" role="img" focusable="false" style="vertical-align: -0.705ex;"><defs><path stroke-width="1" id="E19-MJMATHI-68" d="M137 683Q138 683 209 688T282 694Q294 694 294 685Q294 674 258 534Q220 386 220 383Q220 381 227 388Q288 442 357 442Q411 442 444 415T478 336Q478 285 440 178T402 50Q403 36 407 31T422 26Q450 26 474 56T513 138Q516 149 519 151T535 153Q555 153 555 145Q555 144 551 130Q535 71 500 33Q466 -10 419 -10H414Q367 -10 346 17T325 74Q325 90 361 192T398 345Q398 404 354 404H349Q266 404 205 306L198 293L164 158Q132 28 127 16Q114 -11 83 -11Q69 -11 59 -2T48 16Q48 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xlink:href="#E19-MJMATHI-68" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E19-MJMATHI-3B8" x="815" y="-219"></use><g transform="translate(1175,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E19-MJMAIN-28" x="0" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E19-MJMATHI-78" x="389" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E19-MJMAIN-29" x="962" y="0"></use></g></g></svg></span><script type="math/tex" id="MathJax-Element-18">h_\theta \left( x \right)</script> 的作用是，根据输入 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-19-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="1.33ex" height="1.41ex" viewBox="0 -504.6 572.5 607.1" role="img" focusable="false" style="vertical-align: -0.238ex;"><defs><path stroke-width="1" id="E20-MJMATHI-78" d="M52 289Q59 331 106 386T222 442Q257 442 286 424T329 379Q371 442 430 442Q467 442 494 420T522 361Q522 332 508 314T481 292T458 288Q439 288 427 299T415 328Q415 374 465 391Q454 404 425 404Q412 404 406 402Q368 386 350 336Q290 115 290 78Q290 50 306 38T341 26Q378 26 414 59T463 140Q466 150 469 151T485 153H489Q504 153 504 145Q504 144 502 134Q486 77 440 33T333 -11Q263 -11 227 52Q186 -10 133 -10H127Q78 -10 57 16T35 71Q35 103 54 123T99 143Q142 143 142 101Q142 81 130 66T107 46T94 41L91 40Q91 39 97 36T113 29T132 26Q168 26 194 71Q203 87 217 139T245 247T261 313Q266 340 266 352Q266 380 251 392T217 404Q177 404 142 372T93 290Q91 281 88 280T72 278H58Q52 284 52 289Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E20-MJMATHI-78" x="0" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-19">x</script> 以及参数 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-20-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="1.09ex" height="2.11ex" viewBox="0 -806.1 469.5 908.7" role="img" focusable="false" style="vertical-align: -0.238ex;"><defs><path stroke-width="1" id="E21-MJMATHI-3B8" d="M35 200Q35 302 74 415T180 610T319 704Q320 704 327 704T339 705Q393 701 423 656Q462 596 462 495Q462 380 417 261T302 66T168 -10H161Q125 -10 99 10T60 63T41 130T35 200ZM383 566Q383 668 330 668Q294 668 260 623T204 521T170 421T157 371Q206 370 254 370L351 371Q352 372 359 404T375 484T383 566ZM113 132Q113 26 166 26Q181 26 198 36T239 74T287 161T335 307L340 324H145Q145 321 136 286T120 208T113 132Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E21-MJMATHI-3B8" x="0" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-20">\theta</script>，计算得出”输出 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-21-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="5.416ex" height="2.461ex" viewBox="0 -755.9 2332.1 1059.4" role="img" focusable="false" style="vertical-align: -0.705ex;"><defs><path stroke-width="1" id="E22-MJMATHI-79" d="M21 287Q21 301 36 335T84 406T158 442Q199 442 224 419T250 355Q248 336 247 334Q247 331 231 288T198 191T182 105Q182 62 196 45T238 27Q261 27 281 38T312 61T339 94Q339 95 344 114T358 173T377 247Q415 397 419 404Q432 431 462 431Q475 431 483 424T494 412T496 403Q496 390 447 193T391 -23Q363 -106 294 -155T156 -205Q111 -205 77 -183T43 -117Q43 -95 50 -80T69 -58T89 -48T106 -45Q150 -45 150 -87Q150 -107 138 -122T115 -142T102 -147L99 -148Q101 -153 118 -160T152 -167H160Q177 -167 186 -165Q219 -156 247 -127T290 -65T313 -9T321 21L315 17Q309 13 296 6T270 -6Q250 -11 231 -11Q185 -11 150 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y="-219"></use><g transform="translate(1175,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E24-MJMAIN-28" x="0" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E24-MJMATHI-78" x="389" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E24-MJMAIN-29" x="962" y="0"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E24-MJMAIN-3D" x="2804" y="0"></use><g transform="translate(3860,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E24-MJMAIN-30"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E24-MJMAIN-2E" x="500" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E24-MJMAIN-37" x="779" y="0"></use></g></g></svg></span><script type="math/tex" id="MathJax-Element-23">h_\theta \left( x \right)=0.7</script> 表示病人有 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-24-Frame" tabindex="-1" style="font-size: 100%; display: 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x="5866" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E249-MJMATHI-67" x="6922" y="0"></use><g transform="translate(7569,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E249-MJSZ1-28"></use><g transform="translate(458,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E249-MJMATHI-3B8" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E249-MJMATHI-54" x="663" y="513"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E249-MJMATHI-58" x="1526" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E249-MJSZ1-29" x="2378" y="-1"></use></g></g></svg></span><script type="math/tex" id="MathJax-Element-188">h_\theta \left( x \right)=g(z)=g\left(\theta^{T}X \right)</script>。</p><p>为了得出分类的结果，这里和前面一样，规定以 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-26-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="2.972ex" height="1.994ex" viewBox="0 -755.9 1279.5 858.4" role="img" focusable="false" style="vertical-align: -0.238ex;"><defs><path stroke-width="1" id="E27-MJMAIN-30" d="M96 585Q152 666 249 666Q297 666 345 640T423 548Q460 465 460 320Q460 165 417 83Q397 41 362 16T301 -15T250 -22Q224 -22 198 -16T137 16T82 83Q39 165 39 320Q39 494 96 585ZM321 597Q291 629 250 629Q208 629 178 597Q153 571 145 525T137 333Q137 175 145 125T181 46Q209 16 250 16Q290 16 318 46Q347 76 354 130T362 333Q362 478 354 524T321 597Z"></path><path stroke-width="1" id="E27-MJMAIN-2E" d="M78 60Q78 84 95 102T138 120Q162 120 180 104T199 61Q199 36 182 18T139 0T96 17T78 60Z"></path><path stroke-width="1" id="E27-MJMAIN-35" d="M164 157Q164 133 148 117T109 101H102Q148 22 224 22Q294 22 326 82Q345 115 345 210Q345 313 318 349Q292 382 260 382H254Q176 382 136 314Q132 307 129 306T114 304Q97 304 95 310Q93 314 93 485V614Q93 664 98 664Q100 666 102 666Q103 666 123 658T178 642T253 634Q324 634 389 662Q397 666 402 666Q410 666 410 648V635Q328 538 205 538Q174 538 149 544L139 546V374Q158 388 169 396T205 412T256 420Q337 420 393 355T449 201Q449 109 385 44T229 -22Q148 -22 99 32T50 154Q50 178 61 192T84 210T107 214Q132 214 148 197T164 157Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E27-MJMAIN-30"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E27-MJMAIN-2E" x="500" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E27-MJMAIN-35" x="779" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-26">0.5</script> 为阈值：</p><p><span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-27-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="21.323ex" height="5.846ex" viewBox="0 -1509.8 9180.7 2517" role="img" focusable="false" style="vertical-align: -2.339ex;"><defs><path stroke-width="1" id="E28-MJMATHI-68" d="M137 683Q138 683 209 688T282 694Q294 694 294 685Q294 674 258 534Q220 386 220 383Q220 381 227 388Q288 442 357 442Q411 442 444 415T478 336Q478 285 440 178T402 50Q403 36 407 31T422 26Q450 26 474 56T513 138Q516 149 519 151T535 153Q555 153 555 145Q555 144 551 130Q535 71 500 33Q466 -10 419 -10H414Q367 -10 346 17T325 74Q325 90 361 192T398 345Q398 404 354 404H349Q266 404 205 306L198 293L164 158Q132 28 127 16Q114 -11 83 -11Q69 -11 59 -2T48 16Q48 30 121 320L195 616Q195 629 188 632T149 637H128Q122 643 122 645T124 664Q129 683 137 683Z"></path><path stroke-width="1" id="E28-MJMATHI-3B8" d="M35 200Q35 302 74 415T180 610T319 704Q320 704 327 704T339 705Q393 701 423 656Q462 596 462 495Q462 380 417 261T302 66T168 -10H161Q125 -10 99 10T60 63T41 130T35 200ZM383 566Q383 668 330 668Q294 668 260 623T204 521T170 421T157 371Q206 370 254 370L351 371Q352 372 359 404T375 484T383 566ZM113 132Q113 26 166 26Q181 26 198 36T239 74T287 161T335 307L340 324H145Q145 321 136 286T120 208T113 132Z"></path><path stroke-width="1" id="E28-MJMAIN-28" d="M94 250Q94 319 104 381T127 488T164 576T202 643T244 695T277 729T302 750H315H319Q333 750 333 741Q333 738 316 720T275 667T226 581T184 443T167 250T184 58T225 -81T274 -167T316 -220T333 -241Q333 -250 318 -250H315H302L274 -226Q180 -141 137 -14T94 250Z"></path><path stroke-width="1" id="E28-MJMATHI-78" d="M52 289Q59 331 106 386T222 442Q257 442 286 424T329 379Q371 442 430 442Q467 442 494 420T522 361Q522 332 508 314T481 292T458 288Q439 288 427 299T415 328Q415 374 465 391Q454 404 425 404Q412 404 406 402Q368 386 350 336Q290 115 290 78Q290 50 306 38T341 26Q378 26 414 59T463 140Q466 150 469 151T485 153H489Q504 153 504 145Q504 144 502 134Q486 77 440 33T333 -11Q263 -11 227 52Q186 -10 133 -10H127Q78 -10 57 16T35 71Q35 103 54 123T99 143Q142 143 142 101Q142 81 130 66T107 46T94 41L91 40Q91 39 97 36T113 29T132 26Q168 26 194 71Q203 87 217 139T245 247T261 313Q266 340 266 352Q266 380 251 392T217 404Q177 404 142 372T93 290Q91 281 88 280T72 278H58Q52 284 52 289Z"></path><path stroke-width="1" id="E28-MJMAIN-29" d="M60 749L64 750Q69 750 74 750H86L114 726Q208 641 251 514T294 250Q294 182 284 119T261 12T224 -76T186 -143T145 -194T113 -227T90 -246Q87 -249 86 -250H74Q66 -250 63 -250T58 -247T55 -238Q56 -237 66 -225Q221 -64 221 250T66 725Q56 737 55 738Q55 746 60 749Z"></path><path stroke-width="1" id="E28-MJMAIN-2265" d="M83 616Q83 624 89 630T99 636Q107 636 253 568T543 431T687 361Q694 356 694 346T687 331Q685 329 395 192L107 56H101Q83 58 83 76Q83 77 83 79Q82 86 98 95Q117 105 248 167Q326 204 378 228L626 346L360 472Q291 505 200 548Q112 589 98 597T83 616ZM84 -118Q84 -108 99 -98H678Q694 -104 694 -118Q694 -130 679 -138H98Q84 -131 84 -118Z"></path><path stroke-width="1" id="E28-MJMAIN-30" d="M96 585Q152 666 249 666Q297 666 345 640T423 548Q460 465 460 320Q460 165 417 83Q397 41 362 16T301 -15T250 -22Q224 -22 198 -16T137 16T82 83Q39 165 39 320Q39 494 96 585ZM321 597Q291 629 250 629Q208 629 178 597Q153 571 145 525T137 333Q137 175 145 125T181 46Q209 16 250 16Q290 16 318 46Q347 76 354 130T362 333Q362 478 354 524T321 597Z"></path><path stroke-width="1" id="E28-MJMAIN-2E" d="M78 60Q78 84 95 102T138 120Q162 120 180 104T199 61Q199 36 182 18T139 0T96 17T78 60Z"></path><path stroke-width="1" id="E28-MJMAIN-35" d="M164 157Q164 133 148 117T109 101H102Q148 22 224 22Q294 22 326 82Q345 115 345 210Q345 313 318 349Q292 382 260 382H254Q176 382 136 314Q132 307 129 306T114 304Q97 304 95 310Q93 314 93 485V614Q93 664 98 664Q100 666 102 666Q103 666 123 658T178 642T253 634Q324 634 389 662Q397 666 402 666Q410 666 410 648V635Q328 538 205 538Q174 538 149 544L139 546V374Q158 388 169 396T205 412T256 420Q337 420 393 355T449 201Q449 109 385 44T229 -22Q148 -22 99 32T50 154Q50 178 61 192T84 210T107 214Q132 214 148 197T164 157Z"></path><path stroke-width="1" id="E28-MJMAIN-2192" d="M56 237T56 250T70 270H835Q719 357 692 493Q692 494 692 496T691 499Q691 511 708 511H711Q720 511 723 510T729 506T732 497T735 481T743 456Q765 389 816 336T935 261Q944 258 944 250Q944 244 939 241T915 231T877 212Q836 186 806 152T761 85T740 35T732 4Q730 -6 727 -8T711 -11Q691 -11 691 0Q691 7 696 25Q728 151 835 230H70Q56 237 56 250Z"></path><path stroke-width="1" id="E28-MJMATHI-79" d="M21 287Q21 301 36 335T84 406T158 442Q199 442 224 419T250 355Q248 336 247 334Q247 331 231 288T198 191T182 105Q182 62 196 45T238 27Q261 27 281 38T312 61T339 94Q339 95 344 114T358 173T377 247Q415 397 419 404Q432 431 462 431Q475 431 483 424T494 412T496 403Q496 390 447 193T391 -23Q363 -106 294 -155T156 -205Q111 -205 77 -183T43 -117Q43 -95 50 -80T69 -58T89 -48T106 -45Q150 -45 150 -87Q150 -107 138 -122T115 -142T102 -147L99 -148Q101 -153 118 -160T152 -167H160Q177 -167 186 -165Q219 -156 247 -127T290 -65T313 -9T321 21L315 17Q309 13 296 6T270 -6Q250 -11 231 -11Q185 -11 150 11T104 82Q103 89 103 113Q103 170 138 262T173 379Q173 380 173 381Q173 390 173 393T169 400T158 404H154Q131 404 112 385T82 344T65 302T57 280Q55 278 41 278H27Q21 284 21 287Z"></path><path stroke-width="1" id="E28-MJMAIN-3D" d="M56 347Q56 360 70 367H707Q722 359 722 347Q722 336 708 328L390 327H72Q56 332 56 347ZM56 153Q56 168 72 173H708Q722 163 722 153Q722 140 707 133H70Q56 140 56 153Z"></path><path stroke-width="1" id="E28-MJMAIN-31" d="M213 578L200 573Q186 568 160 563T102 556H83V602H102Q149 604 189 617T245 641T273 663Q275 666 285 666Q294 666 302 660V361L303 61Q310 54 315 52T339 48T401 46H427V0H416Q395 3 257 3Q121 3 100 0H88V46H114Q136 46 152 46T177 47T193 50T201 52T207 57T213 61V578Z"></path><path stroke-width="1" id="E28-MJMAIN-3C" d="M694 -11T694 -19T688 -33T678 -40Q671 -40 524 29T234 166L90 235Q83 240 83 250Q83 261 91 266Q664 540 678 540Q681 540 687 534T694 519T687 505Q686 504 417 376L151 250L417 124Q686 -4 687 -5Q694 -11 694 -19Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><g transform="translate(167,0)"><g transform="translate(-15,0)"><g transform="translate(0,651)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E28-MJMATHI-68" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E28-MJMATHI-3B8" x="815" y="-219"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E28-MJMAIN-28" x="1008" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E28-MJMATHI-78" x="1397" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E28-MJMAIN-29" x="1970" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E28-MJMAIN-2265" x="2637" y="0"></use><g transform="translate(3694,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E28-MJMAIN-30"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E28-MJMAIN-2E" x="500" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E28-MJMAIN-35" x="779" y="0"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E28-MJMAIN-2192" x="5251" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E28-MJMATHI-79" x="6529" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E28-MJMAIN-3D" x="7304" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E28-MJMAIN-31" x="8361" y="0"></use></g><g transform="translate(0,-700)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E28-MJMATHI-68" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E28-MJMATHI-3B8" x="815" y="-219"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E28-MJMAIN-28" x="1008" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E28-MJMATHI-78" x="1397" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E28-MJMAIN-29" x="1970" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E28-MJMAIN-3C" x="2637" y="0"></use><g transform="translate(3694,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E28-MJMAIN-30"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E28-MJMAIN-2E" x="500" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E28-MJMAIN-35" x="779" y="0"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E28-MJMAIN-2192" x="5251" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E28-MJMATHI-79" x="6529" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E28-MJMAIN-3D" x="7304" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E28-MJMAIN-30" x="8361" y="0"></use></g></g></g></g></svg></span><script type="math/tex" id="MathJax-Element-27">\begin{align*}& h_\theta(x) \geq 0.5 \rightarrow y = 1 \newline& h_\theta(x) < 0.5 \rightarrow y = 0 \newline\end{align*}</script></p><p>回忆一下逻辑函数的图像：</p><p><img src='image/2413fbec8ff9fa1f19aaf78265b8a33b_Logistic_function.png' alt='sigmoid function' /></p><p>观察可得当 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-113-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="10.084ex" height="2.577ex" viewBox="0 -806.1 4341.6 1109.7" role="img" focusable="false" style="vertical-align: -0.705ex;"><defs><path stroke-width="1" id="E123-MJMATHI-67" d="M311 43Q296 30 267 15T206 0Q143 0 105 45T66 160Q66 265 143 353T314 442Q361 442 401 394L404 398Q406 401 409 404T418 412T431 419T447 422Q461 422 470 413T480 394Q480 379 423 152T363 -80Q345 -134 286 -169T151 -205Q10 -205 10 -137Q10 -111 28 -91T74 -71Q89 -71 102 -80T116 -111Q116 -121 114 -130T107 -144T99 -154T92 -162L90 -164H91Q101 -167 151 -167Q189 -167 211 -155Q234 -144 254 -122T282 -75Q288 -56 298 -13Q311 35 311 43ZM384 328L380 339Q377 350 375 354T369 368T359 382T346 393T328 402T306 405Q262 405 221 352Q191 313 171 233T151 117Q151 38 213 38Q269 38 323 108L331 118L384 328Z"></path><path stroke-width="1" id="E123-MJMAIN-28" d="M94 250Q94 319 104 381T127 488T164 576T202 643T244 695T277 729T302 750H315H319Q333 750 333 741Q333 738 316 720T275 667T226 581T184 443T167 250T184 58T225 -81T274 -167T316 -220T333 -241Q333 -250 318 -250H315H302L274 -226Q180 -141 137 -14T94 250Z"></path><path stroke-width="1" id="E123-MJMATHI-7A" d="M347 338Q337 338 294 349T231 360Q211 360 197 356T174 346T162 335T155 324L153 320Q150 317 138 317Q117 317 117 325Q117 330 120 339Q133 378 163 406T229 440Q241 442 246 442Q271 442 291 425T329 392T367 375Q389 375 411 408T434 441Q435 442 449 442H462Q468 436 468 434Q468 430 463 420T449 399T432 377T418 358L411 349Q368 298 275 214T160 106L148 94L163 93Q185 93 227 82T290 71Q328 71 360 90T402 140Q406 149 409 151T424 153Q443 153 443 143Q443 138 442 134Q425 72 376 31T278 -11Q252 -11 232 6T193 40T155 57Q111 57 76 -3Q70 -11 59 -11H54H41Q35 -5 35 -2Q35 13 93 84Q132 129 225 214T340 322Q352 338 347 338Z"></path><path stroke-width="1" id="E123-MJMAIN-29" d="M60 749L64 750Q69 750 74 750H86L114 726Q208 641 251 514T294 250Q294 182 284 119T261 12T224 -76T186 -143T145 -194T113 -227T90 -246Q87 -249 86 -250H74Q66 -250 63 -250T58 -247T55 -238Q56 -237 66 -225Q221 -64 221 250T66 725Q56 737 55 738Q55 746 60 749Z"></path><path stroke-width="1" id="E123-MJMAIN-2265" d="M83 616Q83 624 89 630T99 636Q107 636 253 568T543 431T687 361Q694 356 694 346T687 331Q685 329 395 192L107 56H101Q83 58 83 76Q83 77 83 79Q82 86 98 95Q117 105 248 167Q326 204 378 228L626 346L360 472Q291 505 200 548Q112 589 98 597T83 616ZM84 -118Q84 -108 99 -98H678Q694 -104 694 -118Q694 -130 679 -138H98Q84 -131 84 -118Z"></path><path stroke-width="1" id="E123-MJMAIN-30" d="M96 585Q152 666 249 666Q297 666 345 640T423 548Q460 465 460 320Q460 165 417 83Q397 41 362 16T301 -15T250 -22Q224 -22 198 -16T137 16T82 83Q39 165 39 320Q39 494 96 585ZM321 597Q291 629 250 629Q208 629 178 597Q153 571 145 525T137 333Q137 175 145 125T181 46Q209 16 250 16Q290 16 318 46Q347 76 354 130T362 333Q362 478 354 524T321 597Z"></path><path stroke-width="1" id="E123-MJMAIN-2E" d="M78 60Q78 84 95 102T138 120Q162 120 180 104T199 61Q199 36 182 18T139 0T96 17T78 60Z"></path><path stroke-width="1" id="E123-MJMAIN-35" d="M164 157Q164 133 148 117T109 101H102Q148 22 224 22Q294 22 326 82Q345 115 345 210Q345 313 318 349Q292 382 260 382H254Q176 382 136 314Q132 307 129 306T114 304Q97 304 95 310Q93 314 93 485V614Q93 664 98 664Q100 666 102 666Q103 666 123 658T178 642T253 634Q324 634 389 662Q397 666 402 666Q410 666 410 648V635Q328 538 205 538Q174 538 149 544L139 546V374Q158 388 169 396T205 412T256 420Q337 420 393 355T449 201Q449 109 385 44T229 -22Q148 -22 99 32T50 154Q50 178 61 192T84 210T107 214Q132 214 148 197T164 157Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E123-MJMATHI-67" x="0" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E123-MJMAIN-28" x="480" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E123-MJMATHI-7A" x="870" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E123-MJMAIN-29" x="1338" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E123-MJMAIN-2265" x="2005" y="0"></use><g transform="translate(3062,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E123-MJMAIN-30"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E123-MJMAIN-2E" x="500" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E123-MJMAIN-35" x="779" y="0"></use></g></g></svg></span><script type="math/tex" id="MathJax-Element-113">g(z) \geq 0.5</script> 时，有 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-153-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" 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id="MathJax-Element-153">z \geq 0</script>，即 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-211-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="8.721ex" height="2.577ex" viewBox="0 -906.7 3754.7 1109.7" role="img" focusable="false" style="vertical-align: -0.472ex;"><defs><path stroke-width="1" id="E285-MJMATHI-3B8" d="M35 200Q35 302 74 415T180 610T319 704Q320 704 327 704T339 705Q393 701 423 656Q462 596 462 495Q462 380 417 261T302 66T168 -10H161Q125 -10 99 10T60 63T41 130T35 200ZM383 566Q383 668 330 668Q294 668 260 623T204 521T170 421T157 371Q206 370 254 370L351 371Q352 372 359 404T375 484T383 566ZM113 132Q113 26 166 26Q181 26 198 36T239 74T287 161T335 307L340 324H145Q145 321 136 286T120 208T113 132Z"></path><path stroke-width="1" id="E285-MJMATHI-54" d="M40 437Q21 437 21 445Q21 450 37 501T71 602L88 651Q93 669 101 677H569H659Q691 677 697 676T704 667Q704 661 687 553T668 444Q668 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840 637H824Q790 636 763 628T722 611T698 593L687 584Q687 585 592 480L505 384Q505 383 536 304T601 142T638 56Q648 47 699 46Q734 46 734 37Q734 35 732 23Q728 7 725 4T711 1Q708 1 678 1T589 2Q528 2 496 2T461 1Q444 1 444 10Q444 11 446 25Q448 35 450 39T455 44T464 46T480 47T506 54Q523 62 523 64Q522 64 476 181L429 299Q241 95 236 84Q232 76 232 72Q232 53 261 47Q262 47 267 47T273 46Q276 46 277 46T280 45T283 42T284 35Q284 26 282 19Q279 6 276 4T261 1Q258 1 243 1T201 2T142 2Q64 2 42 0Z"></path><path stroke-width="1" id="E285-MJMAIN-2265" d="M83 616Q83 624 89 630T99 636Q107 636 253 568T543 431T687 361Q694 356 694 346T687 331Q685 329 395 192L107 56H101Q83 58 83 76Q83 77 83 79Q82 86 98 95Q117 105 248 167Q326 204 378 228L626 346L360 472Q291 505 200 548Q112 589 98 597T83 616ZM84 -118Q84 -108 99 -98H678Q694 -104 694 -118Q694 -130 679 -138H98Q84 -131 84 -118Z"></path><path stroke-width="1" id="E285-MJMAIN-30" d="M96 585Q152 666 249 666Q297 666 345 640T423 548Q460 465 460 320Q460 165 417 83Q397 41 362 16T301 -15T250 -22Q224 -22 198 -16T137 16T82 83Q39 165 39 320Q39 494 96 585ZM321 597Q291 629 250 629Q208 629 178 597Q153 571 145 525T137 333Q137 175 145 125T181 46Q209 16 250 16Q290 16 318 46Q347 76 354 130T362 333Q362 478 354 524T321 597Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E285-MJMATHI-3B8" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E285-MJMATHI-54" x="663" y="513"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E285-MJMATHI-58" x="1067" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E285-MJMAIN-2265" x="2197" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E285-MJMAIN-30" x="3254" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-211">\theta^TX \geq 0</script>。</p><p>同线性回归模型的不同点在于： <span 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y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E605-MJMATHI-7A" x="10505" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E605-MJMAIN-29" x="10973" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E605-MJMAIN-3D" x="11641" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E605-MJMAIN-30" x="12697" y="0"></use></g></g></g></g></svg></span><script type="math/tex" id="MathJax-Element-433">\begin{align*}z \to +\infty, e^{-\infty} \to 0 \Rightarrow g(z)=1 \newline z \to -\infty, e^{\infty}\to \infty \Rightarrow g(z)=0 \end{align*}</script></p><p>直观一点来个例子，<span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-224-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="29.162ex" height="2.577ex" viewBox="0 -806.1 12555.8 1109.7" role="img" focusable="false" style="vertical-align: -0.705ex;"><defs><path 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y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E299-MJMAIN-32" x="809" y="-213"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E299-MJMAIN-29" x="7658" y="0"></use></g></g></svg></span><script type="math/tex" id="MathJax-Element-224">{h_\theta}\left( x \right)=g\left( {\theta_0}+{\theta_1}{x_1}+{\theta_{2}}{x_{2}}\right)</script> 是下图模型的假设函数。</p><p><img src='image/20180111_000814.png' alt='' /></p><p>根据上面的讨论，要进行分类，那么只要 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-250-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="21.144ex" height="2.461ex" viewBox="0 -806.1 9103.5 1059.4" role="img" focusable="false" style="vertical-align: -0.588ex;"><defs><path stroke-width="1" id="E343-MJMATHI-3B8" d="M35 200Q35 302 74 415T180 610T319 704Q320 704 327 704T339 705Q393 701 423 656Q462 596 462 495Q462 380 417 261T302 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x="809" y="-435"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E297-MJMAIN-2B" x="11122" y="0"></use><g transform="translate(12123,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E297-MJMATHI-3B8" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E297-MJMAIN-34" x="663" y="-213"></use></g><g transform="translate(13046,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E297-MJMATHI-78" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E297-MJMAIN-32" x="809" y="488"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E297-MJMAIN-32" x="809" y="-435"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E297-MJSZ1-29" x="14072" y="-1"></use></g></g></svg></span><script type="math/tex" id="MathJax-Element-222">{h_\theta}\left( x \right)=g\left( {\theta_0}+{\theta_1}{x_1}+{\theta_{2}}{x_{2}}+{\theta_{3}}x_{1}^{2}+{\theta_{4}}x_{2}^{2} \right)</script></p><p>这里取 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-415-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="11.001ex" height="15.651ex" viewBox="0 -3620.7 4736.6 6738.7" role="img" focusable="false" style="vertical-align: -7.242ex;"><defs><path stroke-width="1" id="E584-MJMATHI-3B8" d="M35 200Q35 302 74 415T180 610T319 704Q320 704 327 704T339 705Q393 701 423 656Q462 596 462 495Q462 380 417 261T302 66T168 -10H161Q125 -10 99 10T60 63T41 130T35 200ZM383 566Q383 668 330 668Q294 668 260 623T204 521T170 421T157 371Q206 370 254 370L351 371Q352 372 359 404T375 484T383 566ZM113 132Q113 26 166 26Q181 26 198 36T239 74T287 161T335 307L340 324H145Q145 321 136 286T120 208T113 132Z"></path><path stroke-width="1" id="E584-MJMAIN-3D" d="M56 347Q56 360 70 367H707Q722 359 722 347Q722 336 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transform="translate(0,3550)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E584-MJSZ4-23A1" x="0" y="-1155"></use><g transform="translate(0,-4843.8708609271525) scale(1,5.129139072847682)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E584-MJSZ4-23A2"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E584-MJSZ4-23A3" x="0" y="-5955"></use></g><g transform="translate(834,0)"><g transform="translate(-15,0)"><g transform="translate(0,2750)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E584-MJMAIN-2212" x="0" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E584-MJMAIN-31" x="778" y="0"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E584-MJMAIN-30" x="389" y="1350"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E584-MJMAIN-30" x="389" y="-50"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E584-MJMAIN-31" x="389" y="-1450"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E584-MJMAIN-31" x="389" y="-2850"></use></g></g><g transform="translate(2265,3550)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E584-MJSZ4-23A4" x="0" y="-1155"></use><g transform="translate(0,-4843.8708609271525) scale(1,5.129139072847682)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E584-MJSZ4-23A5"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E584-MJSZ4-23A6" x="0" y="-5955"></use></g></g></g></svg></span><script type="math/tex" id="MathJax-Element-415">\theta = \begin{bmatrix} -1\\0\\0\\1\\1\end{bmatrix}</script>，决策边界对应了一个在原点处的单位圆，如此便可给出分类结果了，如图中品红色曲线：</p><p>&nbsp;</p><p><img src='image/20180111_000653.png' alt='' /></p><p>当然，通过一些更为复杂的多项式，还能拟合那些图像显得非常怪异的数据，使得决策边界像碗状、爱心状等等。</p><p>&nbsp;</p><p>简单来说，决策边界就是<strong>分类的分界线</strong>，分类现在实际就由 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-115-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="1.088ex" height="1.41ex" viewBox="0 -504.6 468.5 607.1" role="img" focusable="false" style="vertical-align: -0.238ex;"><defs><path stroke-width="1" id="E126-MJMATHI-7A" d="M347 338Q337 338 294 349T231 360Q211 360 197 356T174 346T162 335T155 324L153 320Q150 317 138 317Q117 317 117 325Q117 330 120 339Q133 378 163 406T229 440Q241 442 246 442Q271 442 291 425T329 392T367 375Q389 375 411 408T434 441Q435 442 449 442H462Q468 436 468 434Q468 430 463 420T449 399T432 377T418 358L411 349Q368 298 275 214T160 106L148 94L163 93Q185 93 227 82T290 71Q328 71 360 90T402 140Q406 149 409 151T424 153Q443 153 443 143Q443 138 442 134Q425 72 376 31T278 -11Q252 -11 232 6T193 40T155 57Q111 57 76 -3Q70 -11 59 -11H54H41Q35 -5 35 -2Q35 13 93 84Q132 129 225 214T340 322Q352 338 347 338Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E126-MJMATHI-7A" x="0" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-115">z</script> (中的 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-20-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="1.09ex" height="2.11ex" viewBox="0 -806.1 469.5 908.7" role="img" focusable="false" style="vertical-align: -0.238ex;"><defs><path stroke-width="1" id="E21-MJMATHI-3B8" d="M35 200Q35 302 74 415T180 610T319 704Q320 704 327 704T339 705Q393 701 423 656Q462 596 462 495Q462 380 417 261T302 66T168 -10H161Q125 -10 99 10T60 63T41 130T35 200ZM383 566Q383 668 330 668Q294 668 260 623T204 521T170 421T157 371Q206 370 254 370L351 371Q352 372 359 404T375 484T383 566ZM113 132Q113 26 166 26Q181 26 198 36T239 74T287 161T335 307L340 324H145Q145 321 136 286T120 208T113 132Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E21-MJMATHI-3B8" x="0" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-20">\theta</script>)决定啦。</p><h2><a name='header-n291' class='md-header-anchor '></a>6.4 代价函数(Cost Function)</h2><p>&nbsp;</p><h2><a name='header-n292' class='md-header-anchor '></a>6.5 Simplified Cost Function and Gradient Descent</h2><h2><a name='header-n293' class='md-header-anchor '></a>6.6 Advanced Optimization</h2><h2><a name='header-n294' class='md-header-anchor '></a>6.7 Multiclass Classification_ One-vs-all</h2><h1><a name='header-n295' class='md-header-anchor '></a>7 Regularization</h1><h2><a name='header-n296' class='md-header-anchor '></a>7.1 The Problem of Overfitting</h2><h2><a name='header-n297' class='md-header-anchor '></a>7.2 Cost Function</h2><h2><a name='header-n298' class='md-header-anchor '></a>7.3 Regularized Linear Regression</h2><h2><a name='header-n299' class='md-header-anchor '></a>7.4 Regularized Logistic Regression</h2></div>
+<div  id='write'  class = 'is-node show-fences-line-number'><div class='md-toc' mdtype='toc'><p class="md-toc-content"><span class="md-toc-item md-toc-h1" data-ref="n2"><a class="md-toc-inner" style="cursor: pointer;" href="#header-n2">6 逻辑回归(Logistic Regression)</a></span><span class="md-toc-item md-toc-h2" data-ref="n3"><a class="md-toc-inner" style="cursor: pointer;" href="#header-n3">6.1 分类(Classification)</a></span><span class="md-toc-item md-toc-h2" data-ref="n40"><a class="md-toc-inner" style="cursor: pointer;" href="#header-n40">6.2 假设函数表示(Hypothesis Representation)</a></span><span class="md-toc-item md-toc-h2" data-ref="n64"><a class="md-toc-inner" style="cursor: pointer;" href="#header-n64">6.3 决策边界(Decision Boundary)</a></span><span class="md-toc-item md-toc-h2" data-ref="n109"><a class="md-toc-inner" style="cursor: pointer;" href="#header-n109">6.4 损失函数(Cost Function)</a></span><span class="md-toc-item md-toc-h2" data-ref="n136"><a class="md-toc-inner" style="cursor: pointer;" href="#header-n136">6.5 简化的成本函数和梯度下降(Simplified Cost Function and Gradient Descent)</a></span><span class="md-toc-item md-toc-h2" data-ref="n198"><a class="md-toc-inner" style="cursor: pointer;" href="#header-n198">6.6 进阶优化(Advanced Optimization)</a></span><span class="md-toc-item md-toc-h2" data-ref="n201"><a class="md-toc-inner" style="cursor: pointer;" href="#header-n201">6.7 多类别分类: 一对多(Multiclass Classification: One-vs-all)</a></span><span class="md-toc-item md-toc-h1" data-ref="n202"><a class="md-toc-inner" style="cursor: pointer;" href="#header-n202">7 Regularization</a></span><span class="md-toc-item md-toc-h2" data-ref="n203"><a class="md-toc-inner" style="cursor: pointer;" href="#header-n203">7.1 The Problem of Overfitting</a></span><span class="md-toc-item md-toc-h2" data-ref="n204"><a class="md-toc-inner" style="cursor: pointer;" href="#header-n204">7.2 Cost Function</a></span><span class="md-toc-item md-toc-h2" data-ref="n205"><a class="md-toc-inner" style="cursor: pointer;" href="#header-n205">7.3 Regularized Linear Regression</a></span><span class="md-toc-item md-toc-h2" data-ref="n206"><a class="md-toc-inner" style="cursor: pointer;" href="#header-n206">7.4 Regularized Logistic Regression</a></span></p></div><h1><a name='header-n2' class='md-header-anchor '></a>6 逻辑回归(Logistic Regression)</h1><h2><a name='header-n3' class='md-header-anchor '></a>6.1 分类(Classification)</h2><p>在分类问题中，预测的结果是离散值（结果是否属于某一类），逻辑回归算法(Logistic Regression)被用于解决这类分类问题。</p><ul><li>垃圾邮件判断</li><li>金融欺诈判断</li><li>肿瘤诊断</li></ul><p>讨论肿瘤诊断问题：</p><p><img src='image/20180109_144040.png' alt='' /></p><p>肿瘤诊断问题的目的是告诉病人<strong>是否</strong>为恶性肿瘤，是一个<strong>二元分类问题(binary class problems)</strong>，则定义 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-2-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="9.68ex" height="2.577ex" viewBox="0 -806.1 4167.7 1109.7" 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y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E2-MJMAIN-2C" x="2721" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E2-MJMAIN-31" x="3166" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E2-MJMAIN-7D" x="3667" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-2"> y \in\lbrace 0, 1\rbrace</script>，其中 0 表示<strong>负向类(negative class)</strong>，代表恶性肿瘤(&quot;-&quot;)，1 为<strong>正向类(positive class)</strong>，代表良性肿瘤(&quot;+&quot;)。如图，定义最右边的样本为<strong>偏差项</strong>。</p><p>在未加入偏差项时，线性回归算法给出了品红色的拟合直线，若规定</p><p><span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-3-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="11.551ex" height="2.577ex" viewBox="0 -806.1 4973.5 1109.7" role="img" focusable="false" style="vertical-align: -0.705ex;"><defs><path stroke-width="1" id="E4-MJMATHI-68" d="M137 683Q138 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y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E6-MJMAIN-35" x="779" y="0"></use></g></g></svg></span><script type="math/tex" id="MathJax-Element-5">h_\theta(x) \lt 0.5</script> ，预测为 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-58-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="5.416ex" height="2.461ex" viewBox="0 -755.9 2332.1 1059.4" role="img" focusable="false" style="vertical-align: -0.705ex;"><defs><path stroke-width="1" id="E59-MJMATHI-79" d="M21 287Q21 301 36 335T84 406T158 442Q199 442 224 419T250 355Q248 336 247 334Q247 331 231 288T198 191T182 105Q182 62 196 45T238 27Q261 27 281 38T312 61T339 94Q339 95 344 114T358 173T377 247Q415 397 419 404Q432 431 462 431Q475 431 483 424T494 412T496 403Q496 390 447 193T391 -23Q363 -106 294 -155T156 -205Q111 -205 77 -183T43 -117Q43 -95 50 -80T69 -58T89 -48T106 -45Q150 -45 150 -87Q150 -107 138 -122T115 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stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E59-MJMATHI-79" x="0" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E59-MJMAIN-3D" x="775" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E59-MJMAIN-30" x="1831" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-58">y = 0</script>，即负向类。</p><p>即以 0.5 为<strong>阈值</strong>(threshold)，则我们就可以根据线性回归结果，得到相对正确的分类结果 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-7-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="1.155ex" height="1.877ex" viewBox="0 -504.6 497.5 808.1" role="img" focusable="false" style="vertical-align: -0.705ex;"><defs><path stroke-width="1" id="E8-MJMATHI-79" d="M21 287Q21 301 36 335T84 406T158 442Q199 442 224 419T250 355Q248 336 247 334Q247 331 231 288T198 191T182 105Q182 62 196 45T238 27Q261 27 281 38T312 61T339 94Q339 95 344 114T358 173T377 247Q415 397 419 404Q432 431 462 431Q475 431 483 424T494 412T496 403Q496 390 447 193T391 -23Q363 -106 294 -155T156 -205Q111 -205 77 -183T43 -117Q43 -95 50 -80T69 -58T89 -48T106 -45Q150 -45 150 -87Q150 -107 138 -122T115 -142T102 -147L99 -148Q101 -153 118 -160T152 -167H160Q177 -167 186 -165Q219 -156 247 -127T290 -65T313 -9T321 21L315 17Q309 13 296 6T270 -6Q250 -11 231 -11Q185 -11 150 11T104 82Q103 89 103 113Q103 170 138 262T173 379Q173 380 173 381Q173 390 173 393T169 400T158 404H154Q131 404 112 385T82 344T65 302T57 280Q55 278 41 278H27Q21 284 21 287Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E8-MJMATHI-79" x="0" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-7">y</script>。</p><p>&nbsp;</p><p>接下来加入偏差项，线性回归算法给出了靛青色的拟合直线，如果阈值仍然为 0.5，可以看到算法在某些情况下会给出完全错误的结果，对于癌症、肿瘤诊断这类要求预测极其精确的问题，这种情况是无法容忍的。</p><p>不仅如此，线性回归算法的值域为 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-8-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="1.764ex" height="1.994ex" viewBox="0 -755.9 759.5 858.4" role="img" focusable="false" style="vertical-align: -0.238ex;"><defs><path stroke-width="1" id="E9-MJMATHI-52" d="M230 637Q203 637 198 638T193 649Q193 676 204 682Q206 683 378 683Q550 682 564 680Q620 672 658 652T712 606T733 563T739 529Q739 484 710 445T643 385T576 351T538 338L545 333Q612 295 612 223Q612 212 607 162T602 80V71Q602 53 603 43T614 25T640 16Q668 16 686 38T712 85Q717 99 720 102T735 105Q755 105 755 93Q755 75 731 36Q693 -21 641 -21H632Q571 -21 531 4T487 82Q487 109 502 166T517 239Q517 290 474 313Q459 320 449 321T378 323H309L277 193Q244 61 244 59Q244 55 245 54T252 50T269 48T302 46H333Q339 38 339 37T336 19Q332 6 326 0H311Q275 2 180 2Q146 2 117 2T71 2T50 1Q33 1 33 10Q33 12 36 24Q41 43 46 45Q50 46 61 46H67Q94 46 127 49Q141 52 146 61Q149 65 218 339T287 628Q287 635 230 637ZM630 554Q630 586 609 608T523 636Q521 636 500 636T462 637H440Q393 637 386 627Q385 624 352 494T319 361Q319 360 388 360Q466 361 492 367Q556 377 592 426Q608 449 619 486T630 554Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E9-MJMATHI-52" x="0" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-8">R</script>，则当线性回归函数给出诸如 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-9-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="31.626ex" height="2.577ex" viewBox="0 -806.1 13616.8 1109.7" role="img" focusable="false" style="vertical-align: -0.705ex;"><defs><path stroke-width="1" id="E10-MJMATHI-68" d="M137 683Q138 683 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270Q225 304 212 352L210 362L209 498Q208 635 207 640Q195 680 154 696T77 713Q68 713 67 716T65 731Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E11-MJMATHI-79" x="0" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E11-MJMAIN-2208" x="775" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E11-MJMAIN-7B" x="1720" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E11-MJMAIN-30" x="2221" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E11-MJMAIN-2C" x="2721" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E11-MJMAIN-31" x="3166" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E11-MJMAIN-7D" x="3667" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-10">y \in \lbrace 0, 1\rbrace</script>，这显得非常怪异。</p><p>&nbsp;</p><p>区别于线性回归算法，逻辑回归算法是一个分类算法，<strong>其输出值永远在 0 到 1 之间</strong>，即 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-11-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="13.49ex" height="2.577ex" viewBox="0 -806.1 5808.2 1109.7" role="img" focusable="false" style="vertical-align: -0.705ex;"><defs><path stroke-width="1" id="E12-MJMATHI-68" d="M137 683Q138 683 209 688T282 694Q294 694 294 685Q294 674 258 534Q220 386 220 383Q220 381 227 388Q288 442 357 442Q411 442 444 415T478 336Q478 285 440 178T402 50Q403 36 407 31T422 26Q450 26 474 56T513 138Q516 149 519 151T535 153Q555 153 555 145Q555 144 551 130Q535 71 500 33Q466 -10 419 -10H414Q367 -10 346 17T325 74Q325 90 361 192T398 345Q398 404 354 404H349Q266 404 205 306L198 293L164 158Q132 28 127 16Q114 -11 83 -11Q69 -11 59 -2T48 16Q48 30 121 320L195 616Q195 629 188 632T149 637H128Q122 643 122 645T124 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xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E12-MJMAIN-30" x="3972" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E12-MJMAIN-2C" x="4473" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E12-MJMAIN-31" x="4918" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E12-MJMAIN-29" x="5418" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-11">h_\theta(x) \in (0,1)</script>。</p><h2><a name='header-n40' class='md-header-anchor '></a>6.2 假设函数表示(Hypothesis Representation)</h2><p>为了使 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-12-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="13.49ex" height="2.577ex" viewBox="0 -806.1 5808.2 1109.7" role="img" focusable="false" style="vertical-align: -0.705ex;"><defs><path stroke-width="1" id="E13-MJMATHI-68" d="M137 683Q138 683 209 688T282 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xlink:href="#E13-MJMAIN-28" x="1008" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E13-MJMATHI-78" x="1397" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E13-MJMAIN-29" x="1970" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E13-MJMAIN-2208" x="2637" y="0"></use><g transform="translate(3583,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E13-MJMAIN-28" x="0" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E13-MJMAIN-30" x="389" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E13-MJMAIN-2C" x="890" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E13-MJMAIN-31" x="1335" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E13-MJMAIN-29" x="1835" y="0"></use></g></g></svg></span><script type="math/tex" id="MathJax-Element-12">h_\theta(x) \in \left(0, 1\right)</script>，引入逻辑回归模型，定义假设函数</p><div 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transform="translate(458,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E680-MJMATHI-3B8" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E680-MJMATHI-54" x="663" y="583"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E680-MJMATHI-78" x="1526" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E680-MJSZ1-29" x="2098" y="-1"></use></g></g></svg></span></span><script type="math/tex; mode=display" id="MathJax-Element-501">h_\theta \left( x \right)=g(z)=g\left(\theta^{T}x \right)</script></div><p>对比线性回归函数 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-13-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="12.776ex" height="2.811ex" viewBox="0 -906.7 5500.9 1210.2" role="img" focusable="false" style="vertical-align: -0.705ex;"><defs><path stroke-width="1" 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46H83H94Q174 46 189 55Q190 56 191 56Q196 59 201 76T241 233Q258 301 269 344Q339 619 339 625Q339 630 310 630H279Q212 630 191 624Q146 614 121 583T67 467Q60 445 57 441T43 437H40Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E14-MJMATHI-68" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E14-MJMATHI-3B8" x="815" y="-219"></use><g transform="translate(1175,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E14-MJMAIN-28" x="0" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E14-MJMATHI-78" x="389" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E14-MJMAIN-29" x="962" y="0"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E14-MJMAIN-3D" x="2804" y="0"></use><g transform="translate(3860,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E14-MJMATHI-3B8" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E14-MJMATHI-54" x="663" y="513"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E14-MJMATHI-78" x="4928" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-13">h_\theta \left( x \right)=\theta^{T}x</script>，<span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-14-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="1.116ex" height="1.877ex" viewBox="0 -504.6 480.5 808.1" role="img" focusable="false" style="vertical-align: -0.705ex;"><defs><path stroke-width="1" id="E15-MJMATHI-67" d="M311 43Q296 30 267 15T206 0Q143 0 105 45T66 160Q66 265 143 353T314 442Q361 442 401 394L404 398Q406 401 409 404T418 412T431 419T447 422Q461 422 470 413T480 394Q480 379 423 152T363 -80Q345 -134 286 -169T151 -205Q10 -205 10 -137Q10 -111 28 -91T74 -71Q89 -71 102 -80T116 -111Q116 -121 114 -130T107 -144T99 -154T92 -162L90 -164H91Q101 -167 151 -167Q189 -167 211 -155Q234 -144 254 -122T282 -75Q288 -56 298 -13Q311 35 311 43ZM384 328L380 339Q377 350 375 354T369 368T359 382T346 393T328 402T306 405Q262 405 221 352Q191 313 171 233T151 117Q151 38 213 38Q269 38 323 108L331 118L384 328Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E15-MJMATHI-67" x="0" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-14">g</script> 表示逻辑函数(<a href='https://en.wikipedia.org/wiki/Logistic_function' target='_blank'>logistic function</a>)，复合起来，则称为逻辑回归函数。</p><p>逻辑函数是 S 形函数，会将所有实数映射到 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-15-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="5.168ex" height="2.577ex" viewBox="0 -806.1 2225.2 1109.7" role="img" focusable="false" style="vertical-align: -0.705ex;"><defs><path stroke-width="1" id="E16-MJMAIN-28" d="M94 250Q94 319 104 381T127 488T164 576T202 643T244 695T277 729T302 750H315H319Q333 750 333 741Q333 738 316 720T275 667T226 581T184 443T167 250T184 58T225 -81T274 -167T316 -220T333 -241Q333 -250 318 -250H315H302L274 -226Q180 -141 137 -14T94 250Z"></path><path stroke-width="1" id="E16-MJMAIN-30" d="M96 585Q152 666 249 666Q297 666 345 640T423 548Q460 465 460 320Q460 165 417 83Q397 41 362 16T301 -15T250 -22Q224 -22 198 -16T137 16T82 83Q39 165 39 320Q39 494 96 585ZM321 597Q291 629 250 629Q208 629 178 597Q153 571 145 525T137 333Q137 175 145 125T181 46Q209 16 250 16Q290 16 318 46Q347 76 354 130T362 333Q362 478 354 524T321 597Z"></path><path stroke-width="1" id="E16-MJMAIN-2C" d="M78 35T78 60T94 103T137 121Q165 121 187 96T210 8Q210 -27 201 -60T180 -117T154 -158T130 -185T117 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xlink:href="#E16-MJMAIN-30" x="389" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E16-MJMAIN-2C" x="890" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E16-MJMAIN-31" x="1335" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E16-MJMAIN-29" x="1835" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-15">(0, 1)</script> 范围。</p><p><a href='https://en.wikipedia.org/wiki/Sigmoid_function' target='_blank'>sigmoid 函数</a>（如下图）是逻辑函数的特殊情况，其公式为 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-16-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="12.814ex" height="3.395ex" viewBox="0 -956.9 5517.2 1461.5" role="img" focusable="false" style="vertical-align: -1.172ex;"><defs><path stroke-width="1" id="E17-MJMATHI-67" d="M311 43Q296 30 267 15T206 0Q143 0 105 45T66 160Q66 265 143 353T314 442Q361 442 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transform="translate(59,-376)"><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E17-MJMAIN-31" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E17-MJMAIN-2B" x="500" y="0"></use><g transform="translate(904,0)"><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E17-MJMATHI-65" x="0" y="0"></use><g transform="translate(329,204)"><use transform="scale(0.5)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E17-MJMAIN-2212" x="0" y="0"></use><use transform="scale(0.5)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E17-MJMATHI-7A" x="778" y="0"></use></g></g></g></g></g></g></svg></span><script type="math/tex" id="MathJax-Element-16">g\left( z \right)=\frac{1}{1+{{e}^{-z}}}</script>。 </p><p><img src='image/2413fbec8ff9fa1f19aaf78265b8a33b_Logistic_function.png' alt='sigmoid function' /></p><p>应用 sigmoid 函数，则逻辑回归模型：<span 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153H489Q504 153 504 145Q504 144 502 134Q486 77 440 33T333 -11Q263 -11 227 52Q186 -10 133 -10H127Q78 -10 57 16T35 71Q35 103 54 123T99 143Q142 143 142 101Q142 81 130 66T107 46T94 41L91 40Q91 39 97 36T113 29T132 26Q168 26 194 71Q203 87 217 139T245 247T261 313Q266 340 266 352Q266 380 251 392T217 404Q177 404 142 372T93 290Q91 281 88 280T72 278H58Q52 284 52 289Z"></path><path stroke-width="1" id="E18-MJMAIN-29" d="M60 749L64 750Q69 750 74 750H86L114 726Q208 641 251 514T294 250Q294 182 284 119T261 12T224 -76T186 -143T145 -194T113 -227T90 -246Q87 -249 86 -250H74Q66 -250 63 -250T58 -247T55 -238Q56 -237 66 -225Q221 -64 221 250T66 725Q56 737 55 738Q55 746 60 749Z"></path><path stroke-width="1" id="E18-MJMAIN-3D" d="M56 347Q56 360 70 367H707Q722 359 722 347Q722 336 708 328L390 327H72Q56 332 56 347ZM56 153Q56 168 72 173H708Q722 163 722 153Q722 140 707 133H70Q56 140 56 153Z"></path><path stroke-width="1" id="E18-MJMATHI-67" d="M311 43Q296 30 267 15T206 0Q143 0 105 45T66 160Q66 265 143 353T314 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transform="translate(60,-608)"><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E18-MJMAIN-31" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E18-MJMAIN-2B" x="500" y="0"></use><g transform="translate(904,0)"><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E18-MJMATHI-65" x="0" y="0"></use><g transform="translate(329,204)"><use transform="scale(0.5)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E18-MJMAIN-2212" x="0" y="0"></use><g transform="translate(389,0)"><use transform="scale(0.5)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E18-MJMATHI-3B8" x="0" y="0"></use><use transform="scale(0.5)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E18-MJMATHI-54" x="469" y="319"></use></g><use transform="scale(0.5)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E18-MJMATHI-78" x="2052" y="0"></use></g></g></g></g></g></g></svg></span><script type="math/tex" id="MathJax-Element-17">h_{\theta}(x)=g(\theta^Tx) =\frac{1}{1+e^{-\theta^Tx}}</script></p><p>逻辑回归模型中，<span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-18-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="5.868ex" height="2.577ex" viewBox="0 -806.1 2526.7 1109.7" role="img" focusable="false" style="vertical-align: -0.705ex;"><defs><path stroke-width="1" id="E19-MJMATHI-68" d="M137 683Q138 683 209 688T282 694Q294 694 294 685Q294 674 258 534Q220 386 220 383Q220 381 227 388Q288 442 357 442Q411 442 444 415T478 336Q478 285 440 178T402 50Q403 36 407 31T422 26Q450 26 474 56T513 138Q516 149 519 151T535 153Q555 153 555 145Q555 144 551 130Q535 71 500 33Q466 -10 419 -10H414Q367 -10 346 17T325 74Q325 90 361 192T398 345Q398 404 354 404H349Q266 404 205 306L198 293L164 158Q132 28 127 16Q114 -11 83 -11Q69 -11 59 -2T48 16Q48 30 121 320L195 616Q195 629 188 632T149 637H128Q122 643 122 645T124 664Q129 683 137 683Z"></path><path stroke-width="1" id="E19-MJMATHI-3B8" d="M35 200Q35 302 74 415T180 610T319 704Q320 704 327 704T339 705Q393 701 423 656Q462 596 462 495Q462 380 417 261T302 66T168 -10H161Q125 -10 99 10T60 63T41 130T35 200ZM383 566Q383 668 330 668Q294 668 260 623T204 521T170 421T157 371Q206 370 254 370L351 371Q352 372 359 404T375 484T383 566ZM113 132Q113 26 166 26Q181 26 198 36T239 74T287 161T335 307L340 324H145Q145 321 136 286T120 208T113 132Z"></path><path stroke-width="1" id="E19-MJMAIN-28" d="M94 250Q94 319 104 381T127 488T164 576T202 643T244 695T277 729T302 750H315H319Q333 750 333 741Q333 738 316 720T275 667T226 581T184 443T167 250T184 58T225 -81T274 -167T316 -220T333 -241Q333 -250 318 -250H315H302L274 -226Q180 -141 137 -14T94 250Z"></path><path stroke-width="1" id="E19-MJMATHI-78" d="M52 289Q59 331 106 386T222 442Q257 442 286 424T329 379Q371 442 430 442Q467 442 494 420T522 361Q522 332 508 314T481 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xlink:href="#E19-MJMATHI-68" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E19-MJMATHI-3B8" x="815" y="-219"></use><g transform="translate(1175,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E19-MJMAIN-28" x="0" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E19-MJMATHI-78" x="389" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E19-MJMAIN-29" x="962" y="0"></use></g></g></svg></span><script type="math/tex" id="MathJax-Element-18">h_\theta \left( x \right)</script> 的作用是，根据输入 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-19-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="1.33ex" height="1.41ex" viewBox="0 -504.6 572.5 607.1" role="img" focusable="false" style="vertical-align: -0.238ex;"><defs><path stroke-width="1" id="E20-MJMATHI-78" d="M52 289Q59 331 106 386T222 442Q257 442 286 424T329 379Q371 442 430 442Q467 442 494 420T522 361Q522 332 508 314T481 292T458 288Q439 288 427 299T415 328Q415 374 465 391Q454 404 425 404Q412 404 406 402Q368 386 350 336Q290 115 290 78Q290 50 306 38T341 26Q378 26 414 59T463 140Q466 150 469 151T485 153H489Q504 153 504 145Q504 144 502 134Q486 77 440 33T333 -11Q263 -11 227 52Q186 -10 133 -10H127Q78 -10 57 16T35 71Q35 103 54 123T99 143Q142 143 142 101Q142 81 130 66T107 46T94 41L91 40Q91 39 97 36T113 29T132 26Q168 26 194 71Q203 87 217 139T245 247T261 313Q266 340 266 352Q266 380 251 392T217 404Q177 404 142 372T93 290Q91 281 88 280T72 278H58Q52 284 52 289Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E20-MJMATHI-78" x="0" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-19">x</script> 以及参数 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-65-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="1.09ex" height="2.11ex" viewBox="0 -806.1 469.5 908.7" role="img" focusable="false" style="vertical-align: -0.238ex;"><defs><path stroke-width="1" id="E66-MJMATHI-3B8" d="M35 200Q35 302 74 415T180 610T319 704Q320 704 327 704T339 705Q393 701 423 656Q462 596 462 495Q462 380 417 261T302 66T168 -10H161Q125 -10 99 10T60 63T41 130T35 200ZM383 566Q383 668 330 668Q294 668 260 623T204 521T170 421T157 371Q206 370 254 370L351 371Q352 372 359 404T375 484T383 566ZM113 132Q113 26 166 26Q181 26 198 36T239 74T287 161T335 307L340 324H145Q145 321 136 286T120 208T113 132Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E66-MJMATHI-3B8" x="0" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-65">\theta</script>，计算得出”输出 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-51-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="5.416ex" height="2.461ex" viewBox="0 -755.9 2332.1 1059.4" role="img" focusable="false" style="vertical-align: -0.705ex;"><defs><path stroke-width="1" id="E52-MJMATHI-79" d="M21 287Q21 301 36 335T84 406T158 442Q199 442 224 419T250 355Q248 336 247 334Q247 331 231 288T198 191T182 105Q182 62 196 45T238 27Q261 27 281 38T312 61T339 94Q339 95 344 114T358 173T377 247Q415 397 419 404Q432 431 462 431Q475 431 483 424T494 412T496 403Q496 390 447 193T391 -23Q363 -106 294 -155T156 -205Q111 -205 77 -183T43 -117Q43 -95 50 -80T69 -58T89 -48T106 -45Q150 -45 150 -87Q150 -107 138 -122T115 -142T102 -147L99 -148Q101 -153 118 -160T152 -167H160Q177 -167 186 -165Q219 -156 247 -127T290 -65T313 -9T321 21L315 17Q309 13 296 6T270 -6Q250 -11 231 -11Q185 -11 150 11T104 82Q103 89 103 113Q103 170 138 262T173 379Q173 380 173 381Q173 390 173 393T169 400T158 404H154Q131 404 112 385T82 344T65 302T57 280Q55 278 41 278H27Q21 284 21 287Z"></path><path stroke-width="1" id="E52-MJMAIN-3D" d="M56 347Q56 360 70 367H707Q722 359 722 347Q722 336 708 328L390 327H72Q56 332 56 347ZM56 153Q56 168 72 173H708Q722 163 722 153Q722 140 707 133H70Q56 140 56 153Z"></path><path stroke-width="1" id="E52-MJMAIN-31" d="M213 578L200 573Q186 568 160 563T102 556H83V602H102Q149 604 189 617T245 641T273 663Q275 666 285 666Q294 666 302 660V361L303 61Q310 54 315 52T339 48T401 46H427V0H416Q395 3 257 3Q121 3 100 0H88V46H114Q136 46 152 46T177 47T193 50T201 52T207 57T213 61V578Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E52-MJMATHI-79" x="0" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E52-MJMAIN-3D" x="775" y="0"></use><use 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xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E25-MJMAIN-30" x="500" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E25-MJMAIN-25" x="1001" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-24">70\%</script> 的概率得了恶性肿瘤。</p><h2><a name='header-n64' class='md-header-anchor '></a>6.3 决策边界(Decision Boundary)</h2><p>决策边界的概念，可帮助我们更好地理解逻辑回归模型的拟合原理。</p><p>在逻辑回归中，有假设函数 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-25-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="23.521ex" height="3.044ex" viewBox="0 -906.7 10127.1 1310.7" role="img" focusable="false" style="vertical-align: -0.938ex;"><defs><path stroke-width="1" id="E26-MJMATHI-68" d="M137 683Q138 683 209 688T282 694Q294 694 294 685Q294 674 258 534Q220 386 220 383Q220 381 227 388Q288 442 357 442Q411 442 444 415T478 336Q478 285 440 178T402 50Q403 36 407 31T422 26Q450 26 474 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x="5251" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E28-MJMATHI-79" x="6529" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E28-MJMAIN-3D" x="7304" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E28-MJMAIN-31" x="8361" y="0"></use></g><g transform="translate(0,-700)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E28-MJMATHI-68" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E28-MJMATHI-3B8" x="815" y="-219"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E28-MJMAIN-28" x="1008" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E28-MJMATHI-78" x="1397" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E28-MJMAIN-29" x="1970" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E28-MJMAIN-3C" x="2637" y="0"></use><g transform="translate(3694,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E28-MJMAIN-30"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E28-MJMAIN-2E" x="500" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E28-MJMAIN-35" x="779" y="0"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E28-MJMAIN-2192" x="5251" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E28-MJMATHI-79" x="6529" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E28-MJMAIN-3D" x="7304" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E28-MJMAIN-30" x="8361" y="0"></use></g></g></g></g></svg></span><script type="math/tex" id="MathJax-Element-27">\begin{align*}& h_\theta(x) \geq 0.5 \rightarrow y = 1 \newline& h_\theta(x) < 0.5 \rightarrow y = 0 \newline\end{align*}</script></p><p>回忆一下逻辑函数的图像：</p><p><img src='image/2413fbec8ff9fa1f19aaf78265b8a33b_Logistic_function.png' alt='sigmoid function' /></p><p>观察可得当 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-28-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="10.084ex" height="2.577ex" viewBox="0 -806.1 4341.6 1109.7" role="img" focusable="false" style="vertical-align: -0.705ex;"><defs><path stroke-width="1" id="E29-MJMATHI-67" d="M311 43Q296 30 267 15T206 0Q143 0 105 45T66 160Q66 265 143 353T314 442Q361 442 401 394L404 398Q406 401 409 404T418 412T431 419T447 422Q461 422 470 413T480 394Q480 379 423 152T363 -80Q345 -134 286 -169T151 -205Q10 -205 10 -137Q10 -111 28 -91T74 -71Q89 -71 102 -80T116 -111Q116 -121 114 -130T107 -144T99 -154T92 -162L90 -164H91Q101 -167 151 -167Q189 -167 211 -155Q234 -144 254 -122T282 -75Q288 -56 298 -13Q311 35 311 43ZM384 328L380 339Q377 350 375 354T369 368T359 382T346 393T328 402T306 405Q262 405 221 352Q191 313 171 233T151 117Q151 38 213 38Q269 38 323 108L331 118L384 328Z"></path><path stroke-width="1" id="E29-MJMAIN-28" d="M94 250Q94 319 104 381T127 488T164 576T202 643T244 695T277 729T302 750H315H319Q333 750 333 741Q333 738 316 720T275 667T226 581T184 443T167 250T184 58T225 -81T274 -167T316 -220T333 -241Q333 -250 318 -250H315H302L274 -226Q180 -141 137 -14T94 250Z"></path><path stroke-width="1" id="E29-MJMATHI-7A" d="M347 338Q337 338 294 349T231 360Q211 360 197 356T174 346T162 335T155 324L153 320Q150 317 138 317Q117 317 117 325Q117 330 120 339Q133 378 163 406T229 440Q241 442 246 442Q271 442 291 425T329 392T367 375Q389 375 411 408T434 441Q435 442 449 442H462Q468 436 468 434Q468 430 463 420T449 399T432 377T418 358L411 349Q368 298 275 214T160 106L148 94L163 93Q185 93 227 82T290 71Q328 71 360 90T402 140Q406 149 409 151T424 153Q443 153 443 143Q443 138 442 134Q425 72 376 31T278 -11Q252 -11 232 6T193 40T155 57Q111 57 76 -3Q70 -11 59 -11H54H41Q35 -5 35 -2Q35 13 93 84Q132 129 225 214T340 322Q352 338 347 338Z"></path><path stroke-width="1" id="E29-MJMAIN-29" d="M60 749L64 750Q69 750 74 750H86L114 726Q208 641 251 514T294 250Q294 182 284 119T261 12T224 -76T186 -143T145 -194T113 -227T90 -246Q87 -249 86 -250H74Q66 -250 63 -250T58 -247T55 -238Q56 -237 66 -225Q221 -64 221 250T66 725Q56 737 55 738Q55 746 60 749Z"></path><path stroke-width="1" id="E29-MJMAIN-2265" d="M83 616Q83 624 89 630T99 636Q107 636 253 568T543 431T687 361Q694 356 694 346T687 331Q685 329 395 192L107 56H101Q83 58 83 76Q83 77 83 79Q82 86 98 95Q117 105 248 167Q326 204 378 228L626 346L360 472Q291 505 200 548Q112 589 98 597T83 616ZM84 -118Q84 -108 99 -98H678Q694 -104 694 -118Q694 -130 679 -138H98Q84 -131 84 -118Z"></path><path stroke-width="1" id="E29-MJMAIN-30" d="M96 585Q152 666 249 666Q297 666 345 640T423 548Q460 465 460 320Q460 165 417 83Q397 41 362 16T301 -15T250 -22Q224 -22 198 -16T137 16T82 83Q39 165 39 320Q39 494 96 585ZM321 597Q291 629 250 629Q208 629 178 597Q153 571 145 525T137 333Q137 175 145 125T181 46Q209 16 250 16Q290 16 318 46Q347 76 354 130T362 333Q362 478 354 524T321 597Z"></path><path stroke-width="1" id="E29-MJMAIN-2E" d="M78 60Q78 84 95 102T138 120Q162 120 180 104T199 61Q199 36 182 18T139 0T96 17T78 60Z"></path><path stroke-width="1" id="E29-MJMAIN-35" d="M164 157Q164 133 148 117T109 101H102Q148 22 224 22Q294 22 326 82Q345 115 345 210Q345 313 318 349Q292 382 260 382H254Q176 382 136 314Q132 307 129 306T114 304Q97 304 95 310Q93 314 93 485V614Q93 664 98 664Q100 666 102 666Q103 666 123 658T178 642T253 634Q324 634 389 662Q397 666 402 666Q410 666 410 648V635Q328 538 205 538Q174 538 149 544L139 546V374Q158 388 169 396T205 412T256 420Q337 420 393 355T449 201Q449 109 385 44T229 -22Q148 -22 99 32T50 154Q50 178 61 192T84 210T107 214Q132 214 148 197T164 157Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E29-MJMATHI-67" x="0" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E29-MJMAIN-28" x="480" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E29-MJMATHI-7A" x="870" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E29-MJMAIN-29" x="1338" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E29-MJMAIN-2265" x="2005" y="0"></use><g transform="translate(3062,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E29-MJMAIN-30"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E29-MJMAIN-2E" x="500" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E29-MJMAIN-35" x="779" y="0"></use></g></g></svg></span><script type="math/tex" id="MathJax-Element-28">g(z) \geq 0.5</script> 时，有 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-37-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="5.349ex" height="2.227ex" viewBox="0 -755.9 2303.1 958.9" role="img" focusable="false" style="vertical-align: -0.472ex;"><defs><path stroke-width="1" id="E38-MJMATHI-7A" d="M347 338Q337 338 294 349T231 360Q211 360 197 356T174 346T162 335T155 324L153 320Q150 317 138 317Q117 317 117 325Q117 330 120 339Q133 378 163 406T229 440Q241 442 246 442Q271 442 291 425T329 392T367 375Q389 375 411 408T434 441Q435 442 449 442H462Q468 436 468 434Q468 430 463 420T449 399T432 377T418 358L411 349Q368 298 275 214T160 106L148 94L163 93Q185 93 227 82T290 71Q328 71 360 90T402 140Q406 149 409 151T424 153Q443 153 443 143Q443 138 442 134Q425 72 376 31T278 -11Q252 -11 232 6T193 40T155 57Q111 57 76 -3Q70 -11 59 -11H54H41Q35 -5 35 -2Q35 13 93 84Q132 129 225 214T340 322Q352 338 347 338Z"></path><path stroke-width="1" id="E38-MJMAIN-2265" d="M83 616Q83 624 89 630T99 636Q107 636 253 568T543 431T687 361Q694 356 694 346T687 331Q685 329 395 192L107 56H101Q83 58 83 76Q83 77 83 79Q82 86 98 95Q117 105 248 167Q326 204 378 228L626 346L360 472Q291 505 200 548Q112 589 98 597T83 616ZM84 -118Q84 -108 99 -98H678Q694 -104 694 -118Q694 -130 679 -138H98Q84 -131 84 -118Z"></path><path stroke-width="1" id="E38-MJMAIN-30" d="M96 585Q152 666 249 666Q297 666 345 640T423 548Q460 465 460 320Q460 165 417 83Q397 41 362 16T301 -15T250 -22Q224 -22 198 -16T137 16T82 83Q39 165 39 320Q39 494 96 585ZM321 597Q291 629 250 629Q208 629 178 597Q153 571 145 525T137 333Q137 175 145 125T181 46Q209 16 250 16Q290 16 318 46Q347 76 354 130T362 333Q362 478 354 524T321 597Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E38-MJMATHI-7A" x="0" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E38-MJMAIN-2265" x="746" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E38-MJMAIN-30" x="1802" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-37">z \geq 0</script>，即 <span class="MathJax_Preview"></span><span 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380 251 392T217 404Q177 404 142 372T93 290Q91 281 88 280T72 278H58Q52 284 52 289Z"></path><path stroke-width="1" id="E31-MJMAIN-2265" d="M83 616Q83 624 89 630T99 636Q107 636 253 568T543 431T687 361Q694 356 694 346T687 331Q685 329 395 192L107 56H101Q83 58 83 76Q83 77 83 79Q82 86 98 95Q117 105 248 167Q326 204 378 228L626 346L360 472Q291 505 200 548Q112 589 98 597T83 616ZM84 -118Q84 -108 99 -98H678Q694 -104 694 -118Q694 -130 679 -138H98Q84 -131 84 -118Z"></path><path stroke-width="1" id="E31-MJMAIN-30" d="M96 585Q152 666 249 666Q297 666 345 640T423 548Q460 465 460 320Q460 165 417 83Q397 41 362 16T301 -15T250 -22Q224 -22 198 -16T137 16T82 83Q39 165 39 320Q39 494 96 585ZM321 597Q291 629 250 629Q208 629 178 597Q153 571 145 525T137 333Q137 175 145 125T181 46Q209 16 250 16Q290 16 318 46Q347 76 354 130T362 333Q362 478 354 524T321 597Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E31-MJMATHI-3B8" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E31-MJMATHI-54" x="663" y="513"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E31-MJMATHI-78" x="1067" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E31-MJMAIN-2265" x="1917" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E31-MJMAIN-30" x="2974" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-30">\theta^Tx \geq 0</script>。</p><p>同线性回归模型的不同点在于： <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-31-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="31.512ex" height="5.846ex" viewBox="0 -1509.8 13567.4 2517" role="img" focusable="false" style="vertical-align: -2.339ex;"><defs><path stroke-width="1" id="E32-MJMATHI-7A" d="M347 338Q337 338 294 349T231 360Q211 360 197 356T174 346T162 335T155 324L153 320Q150 317 138 317Q117 317 117 325Q117 330 120 339Q133 378 163 406T229 440Q241 442 246 442Q271 442 291 425T329 392T367 375Q389 375 411 408T434 441Q435 442 449 442H462Q468 436 468 434Q468 430 463 420T449 399T432 377T418 358L411 349Q368 298 275 214T160 106L148 94L163 93Q185 93 227 82T290 71Q328 71 360 90T402 140Q406 149 409 151T424 153Q443 153 443 143Q443 138 442 134Q425 72 376 31T278 -11Q252 -11 232 6T193 40T155 57Q111 57 76 -3Q70 -11 59 -11H54H41Q35 -5 35 -2Q35 13 93 84Q132 129 225 214T340 322Q352 338 347 338Z"></path><path stroke-width="1" id="E32-MJMAIN-2192" d="M56 237T56 250T70 270H835Q719 357 692 493Q692 494 692 496T691 499Q691 511 708 511H711Q720 511 723 510T729 506T732 497T735 481T743 456Q765 389 816 336T935 261Q944 258 944 250Q944 244 939 241T915 231T877 212Q836 186 806 152T761 85T740 35T732 4Q730 -6 727 -8T711 -11Q691 -11 691 0Q691 7 696 25Q728 151 835 230H70Q56 237 56 250Z"></path><path stroke-width="1" id="E32-MJMAIN-2B" d="M56 237T56 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{\theta_0}+{\theta_1}{x_1}+{\theta_{2}}{x_{2}}\geq0</script> 时，就预测 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-60-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="5.416ex" height="2.461ex" viewBox="0 -755.9 2332.1 1059.4" role="img" focusable="false" style="vertical-align: -0.705ex;"><defs><path stroke-width="1" id="E61-MJMATHI-79" d="M21 287Q21 301 36 335T84 406T158 442Q199 442 224 419T250 355Q248 336 247 334Q247 331 231 288T198 191T182 105Q182 62 196 45T238 27Q261 27 281 38T312 61T339 94Q339 95 344 114T358 173T377 247Q415 397 419 404Q432 431 462 431Q475 431 483 424T494 412T496 403Q496 390 447 193T391 -23Q363 -106 294 -155T156 -205Q111 -205 77 -183T43 -117Q43 -95 50 -80T69 -58T89 -48T106 -45Q150 -45 150 -87Q150 -107 138 -122T115 -142T102 -147L99 -148Q101 -153 118 -160T152 -167H160Q177 -167 186 -165Q219 -156 247 -127T290 -65T313 -9T321 21L315 17Q309 13 296 6T270 -6Q250 -11 231 -11Q185 -11 150 11T104 82Q103 89 103 113Q103 170 138 262T173 379Q173 380 173 381Q173 390 173 393T169 400T158 404H154Q131 404 112 385T82 344T65 302T57 280Q55 278 41 278H27Q21 284 21 287Z"></path><path stroke-width="1" id="E61-MJMAIN-3D" d="M56 347Q56 360 70 367H707Q722 359 722 347Q722 336 708 328L390 327H72Q56 332 56 347ZM56 153Q56 168 72 173H708Q722 163 722 153Q722 140 707 133H70Q56 140 56 153Z"></path><path stroke-width="1" id="E61-MJMAIN-31" d="M213 578L200 573Q186 568 160 563T102 556H83V602H102Q149 604 189 617T245 641T273 663Q275 666 285 666Q294 666 302 660V361L303 61Q310 54 315 52T339 48T401 46H427V0H416Q395 3 257 3Q121 3 100 0H88V46H114Q136 46 152 46T177 47T193 50T201 52T207 57T213 61V578Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E61-MJMATHI-79" x="0" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E61-MJMAIN-3D" x="775" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E61-MJMAIN-31" x="1831" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-60">y = 1</script>，即预测为正向类。</p><p>如果取 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-35-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="11.001ex" height="9.114ex" viewBox="0 -2213.4 4736.6 3924.2" role="img" focusable="false" style="vertical-align: -3.974ex;"><defs><path stroke-width="1" id="E36-MJMATHI-3B8" d="M35 200Q35 302 74 415T180 610T319 704Q320 704 327 704T339 705Q393 701 423 656Q462 596 462 495Q462 380 417 261T302 66T168 -10H161Q125 -10 99 10T60 63T41 130T35 200ZM383 566Q383 668 330 668Q294 668 260 623T204 521T170 421T157 371Q206 370 254 370L351 371Q352 372 359 404T375 484T383 566ZM113 132Q113 26 166 26Q181 26 198 36T239 74T287 161T335 307L340 324H145Q145 321 136 286T120 208T113 132Z"></path><path stroke-width="1" id="E36-MJMAIN-3D" d="M56 347Q56 360 70 367H707Q722 359 722 347Q722 336 708 328L390 327H72Q56 332 56 347ZM56 153Q56 168 72 173H708Q722 163 722 153Q722 140 707 133H70Q56 140 56 153Z"></path><path stroke-width="1" id="E36-MJMAIN-5B" d="M118 -250V750H255V710H158V-210H255V-250H118Z"></path><path stroke-width="1" id="E36-MJMAIN-2212" d="M84 237T84 250T98 270H679Q694 262 694 250T679 230H98Q84 237 84 250Z"></path><path stroke-width="1" id="E36-MJMAIN-33" d="M127 463Q100 463 85 480T69 524Q69 579 117 622T233 665Q268 665 277 664Q351 652 390 611T430 522Q430 470 396 421T302 350L299 348Q299 347 308 345T337 336T375 315Q457 262 457 175Q457 96 395 37T238 -22Q158 -22 100 21T42 130Q42 158 60 175T105 193Q133 193 151 175T169 130Q169 119 166 110T159 94T148 82T136 74T126 70T118 67L114 66Q165 21 238 21Q293 21 321 74Q338 107 338 175V195Q338 290 274 322Q259 328 213 329L171 330L168 332Q166 335 166 348Q166 366 174 366Q202 366 232 371Q266 376 294 413T322 525V533Q322 590 287 612Q265 626 240 626Q208 626 181 615T143 592T132 580H135Q138 579 143 578T153 573T165 566T175 555T183 540T186 520Q186 498 172 481T127 463Z"></path><path stroke-width="1" id="E36-MJMAIN-31" d="M213 578L200 573Q186 568 160 563T102 556H83V602H102Q149 604 189 617T245 641T273 663Q275 666 285 666Q294 666 302 660V361L303 61Q310 54 315 52T339 48T401 46H427V0H416Q395 3 257 3Q121 3 100 0H88V46H114Q136 46 152 46T177 47T193 50T201 52T207 57T213 61V578Z"></path><path stroke-width="1" id="E36-MJMAIN-5D" d="M22 710V750H159V-250H22V-210H119V710H22Z"></path><path stroke-width="1" id="E36-MJSZ4-23A1" d="M319 -645V1154H666V1070H403V-645H319Z"></path><path stroke-width="1" id="E36-MJSZ4-23A3" d="M319 -644V1155H403V-560H666V-644H319Z"></path><path stroke-width="1" id="E36-MJSZ4-23A2" d="M319 0V602H403V0H319Z"></path><path stroke-width="1" id="E36-MJSZ4-23A4" d="M0 1070V1154H347V-645H263V1070H0Z"></path><path stroke-width="1" id="E36-MJSZ4-23A6" d="M263 -560V1155H347V-644H0V-560H263Z"></path><path stroke-width="1" id="E36-MJSZ4-23A5" d="M263 0V602H347V0H263Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E36-MJMATHI-3B8" x="0" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E36-MJMAIN-3D" x="747" y="0"></use><g transform="translate(1803,0)"><g transform="translate(0,2150)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E36-MJSZ4-23A1" x="0" y="-1155"></use><g transform="translate(0,-2048.5066225165565) scale(1,0.49337748344370863)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E36-MJSZ4-23A2"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E36-MJSZ4-23A3" x="0" y="-3155"></use></g><g transform="translate(834,0)"><g transform="translate(-15,0)"><g transform="translate(0,1350)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E36-MJMAIN-2212" x="0" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E36-MJMAIN-33" x="778" y="0"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E36-MJMAIN-31" x="389" y="-50"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E36-MJMAIN-31" x="389" y="-1450"></use></g></g><g transform="translate(2265,2150)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E36-MJSZ4-23A4" x="0" y="-1155"></use><g transform="translate(0,-2048.5066225165565) scale(1,0.49337748344370863)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E36-MJSZ4-23A5"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E36-MJSZ4-23A6" x="0" y="-3155"></use></g></g></g></svg></span><script type="math/tex" id="MathJax-Element-35">\theta = \begin{bmatrix} -3\\1\\1\end{bmatrix}</script>，则有 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-36-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="17.606ex" height="2.227ex" viewBox="0 -755.9 7580.3 958.9" role="img" focusable="false" style="vertical-align: -0.472ex;"><defs><path stroke-width="1" id="E37-MJMATHI-7A" d="M347 338Q337 338 294 349T231 360Q211 360 197 356T174 346T162 335T155 324L153 320Q150 317 138 317Q117 317 117 325Q117 330 120 339Q133 378 163 406T229 440Q241 442 246 442Q271 442 291 425T329 392T367 375Q389 375 411 408T434 441Q435 442 449 442H462Q468 436 468 434Q468 430 463 420T449 399T432 377T418 358L411 349Q368 298 275 214T160 106L148 94L163 93Q185 93 227 82T290 71Q328 71 360 90T402 140Q406 149 409 151T424 153Q443 153 443 143Q443 138 442 134Q425 72 376 31T278 -11Q252 -11 232 6T193 40T155 57Q111 57 76 -3Q70 -11 59 -11H54H41Q35 -5 35 -2Q35 13 93 84Q132 129 225 214T340 322Q352 338 347 338Z"></path><path stroke-width="1" id="E37-MJMAIN-3D" d="M56 347Q56 360 70 367H707Q722 359 722 347Q722 336 708 328L390 327H72Q56 332 56 347ZM56 153Q56 168 72 173H708Q722 163 722 153Q722 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3V0H50V19V31Q50 38 56 46T86 81Q115 113 136 137Q145 147 170 174T204 211T233 244T261 278T284 308T305 340T320 369T333 401T340 431T343 464Q343 527 309 573T212 619Q179 619 154 602T119 569T109 550Q109 549 114 549Q132 549 151 535T170 489Q170 464 154 447T109 429Z"></path><path stroke-width="1" id="E39-MJMAIN-2265" d="M83 616Q83 624 89 630T99 636Q107 636 253 568T543 431T687 361Q694 356 694 346T687 331Q685 329 395 192L107 56H101Q83 58 83 76Q83 77 83 79Q82 86 98 95Q117 105 248 167Q326 204 378 228L626 346L360 472Q291 505 200 548Q112 589 98 597T83 616ZM84 -118Q84 -108 99 -98H678Q694 -104 694 -118Q694 -130 679 -138H98Q84 -131 84 -118Z"></path><path stroke-width="1" id="E39-MJMAIN-33" d="M127 463Q100 463 85 480T69 524Q69 579 117 622T233 665Q268 665 277 664Q351 652 390 611T430 522Q430 470 396 421T302 350L299 348Q299 347 308 345T337 336T375 315Q457 262 457 175Q457 96 395 37T238 -22Q158 -22 100 21T42 130Q42 158 60 175T105 193Q133 193 151 175T169 130Q169 119 166 110T159 94T148 82T136 74T126 70T118 67L114 66Q165 21 238 21Q293 21 321 74Q338 107 338 175V195Q338 290 274 322Q259 328 213 329L171 330L168 332Q166 335 166 348Q166 366 174 366Q202 366 232 371Q266 376 294 413T322 525V533Q322 590 287 612Q265 626 240 626Q208 626 181 615T143 592T132 580H135Q138 579 143 578T153 573T165 566T175 555T183 540T186 520Q186 498 172 481T127 463Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E39-MJMATHI-78" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E39-MJMAIN-31" x="809" y="-213"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E39-MJMAIN-2B" x="1248" y="0"></use><g transform="translate(2249,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E39-MJMATHI-78" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E39-MJMAIN-32" x="809" y="-213"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E39-MJMAIN-2265" x="3553" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E39-MJMAIN-33" x="4609" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-38">{x_1}+{x_2} \geq 3</script> 时，易绘制图中的品红色直线即<strong>决策边界</strong>，为正向类（以红叉标注的数据）给出 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-51-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="5.416ex" height="2.461ex" viewBox="0 -755.9 2332.1 1059.4" role="img" focusable="false" style="vertical-align: -0.705ex;"><defs><path stroke-width="1" id="E52-MJMATHI-79" d="M21 287Q21 301 36 335T84 406T158 442Q199 442 224 419T250 355Q248 336 247 334Q247 331 231 288T198 191T182 105Q182 62 196 45T238 27Q261 27 281 38T312 61T339 94Q339 95 344 114T358 173T377 247Q415 397 419 404Q432 431 462 431Q475 431 483 424T494 412T496 403Q496 390 447 193T391 -23Q363 -106 294 -155T156 -205Q111 -205 77 -183T43 -117Q43 -95 50 -80T69 -58T89 -48T106 -45Q150 -45 150 -87Q150 -107 138 -122T115 -142T102 -147L99 -148Q101 -153 118 -160T152 -167H160Q177 -167 186 -165Q219 -156 247 -127T290 -65T313 -9T321 21L315 17Q309 13 296 6T270 -6Q250 -11 231 -11Q185 -11 150 11T104 82Q103 89 103 113Q103 170 138 262T173 379Q173 380 173 381Q173 390 173 393T169 400T158 404H154Q131 404 112 385T82 344T65 302T57 280Q55 278 41 278H27Q21 284 21 287Z"></path><path stroke-width="1" id="E52-MJMAIN-3D" d="M56 347Q56 360 70 367H707Q722 359 722 347Q722 336 708 328L390 327H72Q56 332 56 347ZM56 153Q56 168 72 173H708Q722 163 722 153Q722 140 707 133H70Q56 140 56 153Z"></path><path stroke-width="1" id="E52-MJMAIN-31" d="M213 578L200 573Q186 568 160 563T102 556H83V602H102Q149 604 189 617T245 641T273 663Q275 666 285 666Q294 666 302 660V361L303 61Q310 54 315 52T339 48T401 46H427V0H416Q395 3 257 3Q121 3 100 0H88V46H114Q136 46 152 46T177 47T193 50T201 52T207 57T213 61V578Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E52-MJMATHI-79" x="0" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E52-MJMAIN-3D" x="775" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E52-MJMAIN-31" x="1831" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-51">y=1</script> 的分类预测结果。</p><p>&nbsp;</p><p>上面讨论了逻辑回归模型中线性拟合的例子，下面则是一个多项式拟合的例子，和线性回归中的情况也是类似的。</p><p>为了拟合下图数据，建模多项式假设函数：</p><p><span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-40-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="44.221ex" height="3.044ex" viewBox="0 -906.7 19039.3 1310.7" role="img" focusable="false" style="vertical-align: -0.938ex;"><defs><path stroke-width="1" id="E41-MJMATHI-68" d="M137 683Q138 683 209 688T282 694Q294 694 294 685Q294 674 258 534Q220 386 220 383Q220 381 227 388Q288 442 357 442Q411 442 444 415T478 336Q478 285 440 178T402 50Q403 36 407 31T422 26Q450 26 474 56T513 138Q516 149 519 151T535 153Q555 153 555 145Q555 144 551 130Q535 71 500 33Q466 -10 419 -10H414Q367 -10 346 17T325 74Q325 90 361 192T398 345Q398 404 354 404H349Q266 404 205 306L198 293L164 158Q132 28 127 16Q114 -11 83 -11Q69 -11 59 -2T48 16Q48 30 121 320L195 616Q195 629 188 632T149 637H128Q122 643 122 645T124 664Q129 683 137 683Z"></path><path stroke-width="1" id="E41-MJMATHI-3B8" d="M35 200Q35 302 74 415T180 610T319 704Q320 704 327 704T339 705Q393 701 423 656Q462 596 462 495Q462 380 417 261T302 66T168 -10H161Q125 -10 99 10T60 63T41 130T35 200ZM383 566Q383 668 330 668Q294 668 260 623T204 521T170 421T157 371Q206 370 254 370L351 371Q352 372 359 404T375 484T383 566ZM113 132Q113 26 166 26Q181 26 198 36T239 74T287 161T335 307L340 324H145Q145 321 136 286T120 208T113 132Z"></path><path stroke-width="1" 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213 329L171 330L168 332Q166 335 166 348Q166 366 174 366Q202 366 232 371Q266 376 294 413T322 525V533Q322 590 287 612Q265 626 240 626Q208 626 181 615T143 592T132 580H135Q138 579 143 578T153 573T165 566T175 555T183 540T186 520Q186 498 172 481T127 463Z"></path><path stroke-width="1" id="E41-MJMAIN-34" d="M462 0Q444 3 333 3Q217 3 199 0H190V46H221Q241 46 248 46T265 48T279 53T286 61Q287 63 287 115V165H28V211L179 442Q332 674 334 675Q336 677 355 677H373L379 671V211H471V165H379V114Q379 73 379 66T385 54Q393 47 442 46H471V0H462ZM293 211V545L74 212L183 211H293Z"></path><path stroke-width="1" id="E41-MJSZ1-28" d="M152 251Q152 646 388 850H416Q422 844 422 841Q422 837 403 816T357 753T302 649T255 482T236 250Q236 124 255 19T301 -147T356 -251T403 -315T422 -340Q422 -343 416 -349H388Q359 -325 332 -296T271 -213T212 -97T170 56T152 251Z"></path><path stroke-width="1" id="E41-MJSZ1-29" d="M305 251Q305 -145 69 -349H56Q43 -349 39 -347T35 -338Q37 -333 60 -307T108 -239T160 -136T204 27T221 250T204 473T160 636T108 740T60 807T35 839Q35 850 50 850H56H69Q197 743 256 566Q305 425 305 251Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E41-MJMATHI-68" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E41-MJMATHI-3B8" x="815" y="-219"></use><g transform="translate(1175,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E41-MJMAIN-28" x="0" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E41-MJMATHI-78" x="389" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E41-MJMAIN-29" x="962" y="0"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E41-MJMAIN-3D" x="2804" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E41-MJMATHI-67" x="3860" y="0"></use><g transform="translate(4507,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E41-MJSZ1-28"></use><g transform="translate(458,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E41-MJMATHI-3B8" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E41-MJMAIN-30" x="663" y="-213"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E41-MJMAIN-2B" x="1604" y="0"></use><g transform="translate(2604,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E41-MJMATHI-3B8" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E41-MJMAIN-31" x="663" y="-213"></use></g><g transform="translate(3528,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E41-MJMATHI-78" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E41-MJMAIN-31" x="809" y="-213"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E41-MJMAIN-2B" x="4776" y="0"></use><g transform="translate(5777,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E41-MJMATHI-3B8" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E41-MJMAIN-32" x="663" y="-213"></use></g><g transform="translate(6701,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E41-MJMATHI-78" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E41-MJMAIN-32" x="809" y="-213"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E41-MJMAIN-2B" x="7949" y="0"></use><g transform="translate(8950,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E41-MJMATHI-3B8" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E41-MJMAIN-33" x="663" y="-213"></use></g><g transform="translate(9873,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E41-MJMATHI-78" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E41-MJMAIN-32" x="809" y="488"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E41-MJMAIN-31" x="809" y="-435"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E41-MJMAIN-2B" x="11122" y="0"></use><g transform="translate(12123,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E41-MJMATHI-3B8" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E41-MJMAIN-34" x="663" y="-213"></use></g><g transform="translate(13046,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E41-MJMATHI-78" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E41-MJMAIN-32" x="809" y="488"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E41-MJMAIN-32" x="809" y="-435"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E41-MJSZ1-29" x="14072" y="-1"></use></g></g></svg></span><script type="math/tex" id="MathJax-Element-40">{h_\theta}\left( x \right)=g\left( {\theta_0}+{\theta_1}{x_1}+{\theta_{2}}{x_{2}}+{\theta_{3}}x_{1}^{2}+{\theta_{4}}x_{2}^{2} \right)</script></p><p>这里取 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-41-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="11.001ex" height="15.651ex" viewBox="0 -3620.7 4736.6 6738.7" role="img" focusable="false" style="vertical-align: -7.242ex;"><defs><path stroke-width="1" id="E42-MJMATHI-3B8" d="M35 200Q35 302 74 415T180 610T319 704Q320 704 327 704T339 705Q393 701 423 656Q462 596 462 495Q462 380 417 261T302 66T168 -10H161Q125 -10 99 10T60 63T41 130T35 200ZM383 566Q383 668 330 668Q294 668 260 623T204 521T170 421T157 371Q206 370 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xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E42-MJMATHI-3B8" x="0" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E42-MJMAIN-3D" x="747" y="0"></use><g transform="translate(1803,0)"><g transform="translate(0,3550)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E42-MJSZ4-23A1" x="0" y="-1155"></use><g transform="translate(0,-4843.8708609271525) scale(1,5.129139072847682)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E42-MJSZ4-23A2"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E42-MJSZ4-23A3" x="0" y="-5955"></use></g><g transform="translate(834,0)"><g transform="translate(-15,0)"><g transform="translate(0,2750)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E42-MJMAIN-2212" x="0" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E42-MJMAIN-31" x="778" y="0"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E42-MJMAIN-30" x="389" y="1350"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E42-MJMAIN-30" x="389" y="-50"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E42-MJMAIN-31" x="389" y="-1450"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E42-MJMAIN-31" x="389" y="-2850"></use></g></g><g transform="translate(2265,3550)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E42-MJSZ4-23A4" x="0" y="-1155"></use><g transform="translate(0,-4843.8708609271525) scale(1,5.129139072847682)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E42-MJSZ4-23A5"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E42-MJSZ4-23A6" x="0" y="-5955"></use></g></g></g></svg></span><script type="math/tex" id="MathJax-Element-41">\theta = \begin{bmatrix} -1\\0\\0\\1\\1\end{bmatrix}</script>，决策边界对应了一个在原点处的单位圆（<span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-496-Frame" tabindex="-1" style="font-size: 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transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E674-MJMAIN-32" x="1451" y="513"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E674-MJMAIN-3D" x="4461" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E674-MJMAIN-31" x="5517" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-496">{x_1}^2+{x_2}^2 = 1</script>），如此便可给出分类结果，如图中品红色曲线：</p><p>&nbsp;</p><p><img src='image/20180111_000653.png' alt='' /></p><p>当然，通过一些更为复杂的多项式，还能拟合那些图像显得非常怪异的数据，使得决策边界形似碗状、爱心状等等。</p><p>&nbsp;</p><p>简单来说，决策边界就是<strong>分类的分界线</strong>，分类现在实际就由 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-42-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="1.088ex" height="1.41ex" viewBox="0 -504.6 468.5 607.1" role="img" focusable="false" style="vertical-align: -0.238ex;"><defs><path stroke-width="1" id="E43-MJMATHI-7A" d="M347 338Q337 338 294 349T231 360Q211 360 197 356T174 346T162 335T155 324L153 320Q150 317 138 317Q117 317 117 325Q117 330 120 339Q133 378 163 406T229 440Q241 442 246 442Q271 442 291 425T329 392T367 375Q389 375 411 408T434 441Q435 442 449 442H462Q468 436 468 434Q468 430 463 420T449 399T432 377T418 358L411 349Q368 298 275 214T160 106L148 94L163 93Q185 93 227 82T290 71Q328 71 360 90T402 140Q406 149 409 151T424 153Q443 153 443 143Q443 138 442 134Q425 72 376 31T278 -11Q252 -11 232 6T193 40T155 57Q111 57 76 -3Q70 -11 59 -11H54H41Q35 -5 35 -2Q35 13 93 84Q132 129 225 214T340 322Q352 338 347 338Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E43-MJMATHI-7A" x="0" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-42">z</script> (中的 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-65-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="1.09ex" height="2.11ex" viewBox="0 -806.1 469.5 908.7" role="img" focusable="false" style="vertical-align: -0.238ex;"><defs><path stroke-width="1" id="E66-MJMATHI-3B8" d="M35 200Q35 302 74 415T180 610T319 704Q320 704 327 704T339 705Q393 701 423 656Q462 596 462 495Q462 380 417 261T302 66T168 -10H161Q125 -10 99 10T60 63T41 130T35 200ZM383 566Q383 668 330 668Q294 668 260 623T204 521T170 421T157 371Q206 370 254 370L351 371Q352 372 359 404T375 484T383 566ZM113 132Q113 26 166 26Q181 26 198 36T239 74T287 161T335 307L340 324H145Q145 321 136 286T120 208T113 132Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E66-MJMATHI-3B8" x="0" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-65">\theta</script>)决定啦。</p><h2><a name='header-n109' class='md-header-anchor '></a>6.4 损失函数(Cost Function)</h2><p>上节又留下了个问题，我们怎么知道决策边界是啥样？<span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-65-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="1.09ex" height="2.11ex" viewBox="0 -806.1 469.5 908.7" role="img" focusable="false" style="vertical-align: -0.238ex;"><defs><path stroke-width="1" id="E66-MJMATHI-3B8" d="M35 200Q35 302 74 415T180 610T319 704Q320 704 327 704T339 705Q393 701 423 656Q462 596 462 495Q462 380 417 261T302 66T168 -10H161Q125 -10 99 10T60 63T41 130T35 200ZM383 566Q383 668 330 668Q294 668 260 623T204 521T170 421T157 371Q206 370 254 370L351 371Q352 372 359 404T375 484T383 566ZM113 132Q113 26 166 26Q181 26 198 36T239 74T287 161T335 307L340 324H145Q145 321 136 286T120 208T113 132Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use 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xlink:href="#E47-MJMATHI-6D" x="372" y="1344"></use></g><g transform="translate(6199,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E47-MJSZ1-28"></use><g transform="translate(458,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E47-MJMATHI-68" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E47-MJMATHI-3B8" x="815" y="-219"></use></g><g transform="translate(1633,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E47-MJSZ1-28"></use><g transform="translate(458,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E47-MJMATHI-78" x="0" y="0"></use><g transform="translate(572,362)"><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E47-MJMAIN-28" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E47-MJMATHI-69" x="389" y="0"></use><use transform="scale(0.707)" 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xlink:href="#E47-MJMAIN-32" x="10033" y="878"></use></g></g></svg></span><script type="math/tex" id="MathJax-Element-46">J\left( {\theta} \right)=\frac{1}{2m}\sum\limits_{i=1}^{m}{{{\left( h_{\theta} \left({x}^{\left( i \right)} \right)-{y}^{\left( i \right)} \right)}^{2}}}</script></p><p>其中 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-68-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="16.022ex" height="3.044ex" viewBox="0 -906.7 6898.4 1310.7" role="img" focusable="false" style="vertical-align: -0.938ex;"><defs><path stroke-width="1" id="E69-MJMATHI-68" d="M137 683Q138 683 209 688T282 694Q294 694 294 685Q294 674 258 534Q220 386 220 383Q220 381 227 388Q288 442 357 442Q411 442 444 415T478 336Q478 285 440 178T402 50Q403 36 407 31T422 26Q450 26 474 56T513 138Q516 149 519 151T535 153Q555 153 555 145Q555 144 551 130Q535 71 500 33Q466 -10 419 -10H414Q367 -10 346 17T325 74Q325 90 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xlink:href="#E69-MJMATHI-3B8" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E69-MJMATHI-54" x="663" y="513"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E69-MJMATHI-78" x="1526" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E69-MJSZ1-29" x="2098" y="-1"></use></g></g></svg></span><script type="math/tex" id="MathJax-Element-68">h_\theta(x) = g\left(\theta^{T}x \right)</script>，可绘制关于 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-50-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="4.371ex" height="2.577ex" viewBox="0 -806.1 1882 1109.7" role="img" focusable="false" style="vertical-align: -0.705ex;"><defs><path stroke-width="1" id="E51-MJMATHI-4A" d="M447 625Q447 637 354 637H329Q323 642 323 645T325 664Q329 677 335 683H352Q393 681 498 681Q541 681 568 681T605 682T619 682Q633 682 633 672Q633 670 630 658Q626 642 623 640T604 637Q552 637 545 623Q541 610 483 376Q420 128 419 127Q397 64 333 21T195 -22Q137 -22 97 8T57 88Q57 130 80 152T132 174Q177 174 182 130Q182 98 164 80T123 56Q115 54 115 53T122 44Q148 15 197 15Q235 15 271 47T324 130Q328 142 387 380T447 625Z"></path><path stroke-width="1" id="E51-MJMAIN-28" d="M94 250Q94 319 104 381T127 488T164 576T202 643T244 695T277 729T302 750H315H319Q333 750 333 741Q333 738 316 720T275 667T226 581T184 443T167 250T184 58T225 -81T274 -167T316 -220T333 -241Q333 -250 318 -250H315H302L274 -226Q180 -141 137 -14T94 250Z"></path><path stroke-width="1" id="E51-MJMATHI-3B8" d="M35 200Q35 302 74 415T180 610T319 704Q320 704 327 704T339 705Q393 701 423 656Q462 596 462 495Q462 380 417 261T302 66T168 -10H161Q125 -10 99 10T60 63T41 130T35 200ZM383 566Q383 668 330 668Q294 668 260 623T204 521T170 421T157 371Q206 370 254 370L351 371Q352 372 359 404T375 484T383 566ZM113 132Q113 26 166 26Q181 26 198 36T239 74T287 161T335 307L340 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/></p><p>回忆线性回归中的损失函数，其是一个二次凸函数（碗状），二次凸函数的重要性质是只有一个局部最小点即全局最小点。上图中有许多局部最小点，这样梯度下降算法将无法确定收敛点是全局最优。</p><p><img src='image/20180111_080514.png' alt='' /></p><p>如果是一个凸函数，可以对其进行最优化分析，这类最优化问题，称为<strong>凸优化问题</strong>。还好，损失函数不止平方损失函数一种。</p><p>对于逻辑回归，更换平方损失函数为<strong>对数损失函数：</strong></p><p><span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-49-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="47.03ex" height="13.083ex" viewBox="0 -3067.8 20249.2 5633" role="img" focusable="false" style="vertical-align: -5.958ex;"><defs><path stroke-width="1" id="E50-MJMATHI-4A" d="M447 625Q447 637 354 637H329Q323 642 323 645T325 664Q329 677 335 683H352Q393 681 498 681Q541 681 568 681T605 682T619 682Q633 682 633 672Q633 670 630 658Q626 642 623 640T604 637Q552 637 545 623Q541 610 483 376Q420 128 419 127Q397 64 333 21T195 -22Q137 -22 97 8T57 88Q57 130 80 152T132 174Q177 174 182 130Q182 98 164 80T123 56Q115 54 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xlink:href="#E50-MJMAIN-29" x="5699" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E50-MJMAIN-3D" x="6366" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E50-MJMAIN-2212" x="7422" y="0"></use><g transform="translate(8367,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E50-MJMAIN-6C"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E50-MJMAIN-6F" x="278" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E50-MJMAIN-67" x="779" y="0"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E50-MJMAIN-28" x="9647" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E50-MJMAIN-31" x="10036" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E50-MJMAIN-2212" x="10759" y="0"></use><g transform="translate(11760,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E50-MJMATHI-68" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E50-MJMATHI-3B8" x="815" y="-219"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E50-MJMAIN-28" x="12768" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E50-MJMATHI-78" x="13158" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E50-MJMAIN-29" x="13730" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E50-MJMAIN-29" x="14120" y="0"></use></g></g><g transform="translate(16772,0)"><g transform="translate(0,-881)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E50-MJMAIN-69"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E50-MJMAIN-66" x="278" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E50-MJMAIN-79" x="835" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E50-MJMAIN-3D" x="1613" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E50-MJMAIN-31" x="2642" y="0"></use></g><g transform="translate(0,-2232)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E50-MJMAIN-69"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E50-MJMAIN-66" x="278" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E50-MJMAIN-79" x="835" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E50-MJMAIN-3D" x="1613" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E50-MJMAIN-30" x="2642" y="0"></use></g></g></g></g></svg></span><script type="math/tex" id="MathJax-Element-49">\begin{align*}& J(\theta) = \dfrac{1}{m} \sum_{i=1}^m \mathrm{Cost}(h_\theta(x^{(i)}),y^{(i)}) \newline & \mathrm{Cost}(h_\theta(x),y) = -\log(h_\theta(x)) \; & \text{if y = 1} \newline & \mathrm{Cost}(h_\theta(x),y) = -\log(1-h_\theta(x)) \; & \text{if y = 0}\end{align*}</script></p><p>则可绘制关于 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-50-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="4.371ex" height="2.577ex" viewBox="0 -806.1 1882 1109.7" role="img" focusable="false" style="vertical-align: -0.705ex;"><defs><path stroke-width="1" id="E51-MJMATHI-4A" d="M447 625Q447 637 354 637H329Q323 642 323 645T325 664Q329 677 335 683H352Q393 681 498 681Q541 681 568 681T605 682T619 682Q633 682 633 672Q633 670 630 658Q626 642 623 640T604 637Q552 637 545 623Q541 610 483 376Q420 128 419 127Q397 64 333 21T195 -22Q137 -22 97 8T57 88Q57 130 80 152T132 174Q177 174 182 130Q182 98 164 80T123 56Q115 54 115 53T122 44Q148 15 197 15Q235 15 271 47T324 130Q328 142 387 380T447 625Z"></path><path stroke-width="1" id="E51-MJMAIN-28" d="M94 250Q94 319 104 381T127 488T164 576T202 643T244 695T277 729T302 750H315H319Q333 750 333 741Q333 738 316 720T275 667T226 581T184 443T167 250T184 58T225 -81T274 -167T316 -220T333 -241Q333 -250 318 -250H315H302L274 -226Q180 -141 137 -14T94 250Z"></path><path stroke-width="1" id="E51-MJMATHI-3B8" d="M35 200Q35 302 74 415T180 610T319 704Q320 704 327 704T339 705Q393 701 423 656Q462 596 462 495Q462 380 417 261T302 66T168 -10H161Q125 -10 99 10T60 63T41 130T35 200ZM383 566Q383 668 330 668Q294 668 260 623T204 521T170 421T157 371Q206 370 254 370L351 371Q352 372 359 404T375 484T383 566ZM113 132Q113 26 166 26Q181 26 198 36T239 74T287 161T335 307L340 324H145Q145 321 136 286T120 208T113 132Z"></path><path stroke-width="1" id="E51-MJMAIN-29" d="M60 749L64 750Q69 750 74 750H86L114 726Q208 641 251 514T294 250Q294 182 284 119T261 12T224 -76T186 -143T145 -194T113 -227T90 -246Q87 -249 86 -250H74Q66 -250 63 -250T58 -247T55 -238Q56 -237 66 -225Q221 -64 221 250T66 725Q56 737 55 738Q55 746 60 749Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E51-MJMATHI-4A" x="0" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E51-MJMAIN-28" x="633" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E51-MJMATHI-3B8" x="1023" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E51-MJMAIN-29" x="1492" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-50">J(\theta)</script> 的图像如下：</p><p><img src='image/20180111_080614.png' alt='' /></p><p>如左图，当训练集的结果为 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-51-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="5.416ex" height="2.461ex" viewBox="0 -755.9 2332.1 1059.4" role="img" focusable="false" style="vertical-align: -0.705ex;"><defs><path stroke-width="1" id="E52-MJMATHI-79" d="M21 287Q21 301 36 335T84 406T158 442Q199 442 224 419T250 355Q248 336 247 334Q247 331 231 288T198 191T182 105Q182 62 196 45T238 27Q261 27 281 38T312 61T339 94Q339 95 344 114T358 173T377 247Q415 397 419 404Q432 431 462 431Q475 431 483 424T494 412T496 403Q496 390 447 193T391 -23Q363 -106 294 -155T156 -205Q111 -205 77 -183T43 -117Q43 -95 50 -80T69 -58T89 -48T106 -45Q150 -45 150 -87Q150 -107 138 -122T115 -142T102 -147L99 -148Q101 -153 118 -160T152 -167H160Q177 -167 186 -165Q219 -156 247 -127T290 -65T313 -9T321 21L315 17Q309 13 296 6T270 -6Q250 -11 231 -11Q185 -11 150 11T104 82Q103 89 103 113Q103 170 138 262T173 379Q173 380 173 381Q173 390 173 393T169 400T158 404H154Q131 404 112 385T82 344T65 302T57 280Q55 278 41 278H27Q21 284 21 287Z"></path><path stroke-width="1" id="E52-MJMAIN-3D" d="M56 347Q56 360 70 367H707Q722 359 722 347Q722 336 708 328L390 327H72Q56 332 56 347ZM56 153Q56 168 72 173H708Q722 163 722 153Q722 140 707 133H70Q56 140 56 153Z"></path><path stroke-width="1" id="E52-MJMAIN-31" d="M213 578L200 573Q186 568 160 563T102 556H83V602H102Q149 604 189 617T245 641T273 663Q275 666 285 666Q294 666 302 660V361L303 61Q310 54 315 52T339 48T401 46H427V0H416Q395 3 257 3Q121 3 100 0H88V46H114Q136 46 152 46T177 47T193 50T201 52T207 57T213 61V578Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E52-MJMATHI-79" x="0" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E52-MJMAIN-3D" x="775" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E52-MJMAIN-31" x="1831" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-51">y=1</script>（正样本）时，随着假设函数趋向于 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-52-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="1.162ex" height="1.994ex" viewBox="0 -755.9 500.5 858.4" role="img" focusable="false" style="vertical-align: -0.238ex;"><defs><path stroke-width="1" id="E53-MJMAIN-31" d="M213 578L200 573Q186 568 160 563T102 556H83V602H102Q149 604 189 617T245 641T273 663Q275 666 285 666Q294 666 302 660V361L303 61Q310 54 315 52T339 48T401 46H427V0H416Q395 3 257 3Q121 3 100 0H88V46H114Q136 46 152 46T177 47T193 50T201 52T207 57T213 61V578Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E53-MJMAIN-31" x="0" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-52">1</script>，损失函数的值会趋于 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-59-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="1.162ex" height="1.994ex" viewBox="0 -755.9 500.5 858.4" role="img" focusable="false" style="vertical-align: -0.238ex;"><defs><path stroke-width="1" id="E60-MJMAIN-30" d="M96 585Q152 666 249 666Q297 666 345 640T423 548Q460 465 460 320Q460 165 417 83Q397 41 362 16T301 -15T250 -22Q224 -22 198 -16T137 16T82 83Q39 165 39 320Q39 494 96 585ZM321 597Q291 629 250 629Q208 629 178 597Q153 571 145 525T137 333Q137 175 145 125T181 46Q209 16 250 16Q290 16 318 46Q347 76 354 130T362 333Q362 478 354 524T321 597Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E60-MJMAIN-30" x="0" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-59">0</script>，即意味着拟合程度很好。如果假设函数此时趋于 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-59-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="1.162ex" height="1.994ex" viewBox="0 -755.9 500.5 858.4" role="img" focusable="false" style="vertical-align: -0.238ex;"><defs><path stroke-width="1" id="E60-MJMAIN-30" d="M96 585Q152 666 249 666Q297 666 345 640T423 548Q460 465 460 320Q460 165 417 83Q397 41 362 16T301 -15T250 -22Q224 -22 198 -16T137 16T82 83Q39 165 39 320Q39 494 96 585ZM321 597Q291 629 250 629Q208 629 178 597Q153 571 145 525T137 333Q137 175 145 125T181 46Q209 16 250 16Q290 16 318 46Q347 76 354 130T362 333Q362 478 354 524T321 597Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E60-MJMAIN-30" x="0" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-59">0</script>，则会给出一个<strong>很高的损失</strong>，拟合程度<strong>差</strong>，算法会根据其迅速纠正 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-65-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="1.09ex" height="2.11ex" viewBox="0 -806.1 469.5 908.7" role="img" focusable="false" style="vertical-align: -0.238ex;"><defs><path stroke-width="1" id="E66-MJMATHI-3B8" d="M35 200Q35 302 74 415T180 610T319 704Q320 704 327 704T339 705Q393 701 423 656Q462 596 462 495Q462 380 417 261T302 66T168 -10H161Q125 -10 99 10T60 63T41 130T35 200ZM383 566Q383 668 330 668Q294 668 260 623T204 521T170 421T157 371Q206 370 254 370L351 371Q352 372 359 404T375 484T383 566ZM113 132Q113 26 166 26Q181 26 198 36T239 74T287 161T335 307L340 324H145Q145 321 136 286T120 208T113 132Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E66-MJMATHI-3B8" x="0" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-65">\theta</script> 值，右图 <span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-56-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="5.416ex" height="2.461ex" viewBox="0 -755.9 2332.1 1059.4" role="img" focusable="false" style="vertical-align: -0.705ex;"><defs><path stroke-width="1" id="E57-MJMATHI-79" d="M21 287Q21 301 36 335T84 406T158 442Q199 442 224 419T250 355Q248 336 247 334Q247 331 231 288T198 191T182 105Q182 62 196 45T238 27Q261 27 281 38T312 61T339 94Q339 95 344 114T358 173T377 247Q415 397 419 404Q432 431 462 431Q475 431 483 424T494 412T496 403Q496 390 447 193T391 -23Q363 -106 294 -155T156 -205Q111 -205 77 -183T43 -117Q43 -95 50 -80T69 -58T89 -48T106 -45Q150 -45 150 -87Q150 -107 138 -122T115 -142T102 -147L99 -148Q101 -153 118 -160T152 -167H160Q177 -167 186 -165Q219 -156 247 -127T290 -65T313 -9T321 21L315 17Q309 13 296 6T270 -6Q250 -11 231 -11Q185 -11 150 11T104 82Q103 89 103 113Q103 170 138 262T173 379Q173 380 173 381Q173 390 173 393T169 400T158 404H154Q131 404 112 385T82 344T65 302T57 280Q55 278 41 278H27Q21 284 21 287Z"></path><path stroke-width="1" id="E57-MJMAIN-3D" d="M56 347Q56 360 70 367H707Q722 359 722 347Q722 336 708 328L390 327H72Q56 332 56 347ZM56 153Q56 168 72 173H708Q722 163 722 153Q722 140 707 133H70Q56 140 56 153Z"></path><path stroke-width="1" id="E57-MJMAIN-30" d="M96 585Q152 666 249 666Q297 666 345 640T423 548Q460 465 460 320Q460 165 417 83Q397 41 362 16T301 -15T250 -22Q224 -22 198 -16T137 16T82 83Q39 165 39 320Q39 494 96 585ZM321 597Q291 629 250 629Q208 629 178 597Q153 571 145 525T137 333Q137 175 145 125T181 46Q209 16 250 16Q290 16 318 46Q347 76 354 130T362 333Q362 478 354 524T321 597Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="matrix(1 0 0 -1 0 0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E57-MJMATHI-79" x="0" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E57-MJMAIN-3D" x="775" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E57-MJMAIN-30" x="1831" y="0"></use></g></svg></span><script type="math/tex" id="MathJax-Element-56">y=0</script> 同理。</p><p>区别于平方损失函数，对数损失函数也是一个凸函数，但没有局部最优值。</p><h2><a name='header-n136' class='md-header-anchor '></a>6.5 简化的成本函数和梯度下降(Simplified Cost Function and Gradient Descent)</h2><p>由于懒得分类讨论，对于二元分类问题，我们把损失函数<strong>简化</strong>为一个函数： 
+<span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-57-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="63.022ex" height="2.577ex" viewBox="0 -806.1 27134.6 1109.7" role="img" focusable="false" style="vertical-align: -0.705ex;"><defs><path stroke-width="1" id="E58-MJMATHI-43" d="M50 252Q50 367 117 473T286 641T490 704Q580 704 633 653Q642 643 648 636T656 626L657 623Q660 623 684 649Q691 655 699 663T715 679T725 690L740 705H746Q760 705 760 698Q760 694 728 561Q692 422 692 421Q690 416 687 415T669 413H653Q647 419 647 422Q647 423 648 429T650 449T651 481Q651 552 619 605T510 659Q484 659 454 652T382 628T299 572T226 479Q194 422 175 346T156 222Q156 108 232 58Q280 24 350 24Q441 24 512 92T606 240Q610 253 612 255T628 257Q648 257 648 248Q648 243 647 239Q618 132 523 55T319 -22Q206 -22 128 53T50 252Z"></path><path stroke-width="1" id="E58-MJMATHI-6F" d="M201 -11Q126 -11 80 38T34 156Q34 221 64 279T146 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y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E69-MJMAIN-3D" x="2637" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E69-MJMATHI-67" x="3694" y="0"></use><g transform="translate(4341,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E69-MJSZ1-28"></use><g transform="translate(458,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E69-MJMATHI-3B8" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E69-MJMATHI-54" x="663" y="513"></use></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E69-MJMATHI-78" x="1526" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E69-MJSZ1-29" x="2098" y="-1"></use></g></g></svg></span><script type="math/tex" id="MathJax-Element-68">h_\theta(x) = g\left(\theta^{T}x \right)</script>，不过求导后的结果也相同。</p><p>向量化实现：<span class="MathJax_Preview"></span><span class="MathJax_SVG" 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xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E72-MJMAIN-6C"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E72-MJMAIN-6F" x="278" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E72-MJMAIN-67" x="779" y="0"></use></g><g transform="translate(17155,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E72-MJSZ1-28"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E72-MJMAIN-31" x="458" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E72-MJMAIN-2212" x="1181" y="0"></use><g transform="translate(2181,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E72-MJMATHI-68" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E72-MJMATHI-3B8" x="815" y="-219"></use></g><g transform="translate(3357,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E72-MJSZ1-28"></use><g transform="translate(458,0)"><use 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transform="translate(572,521)"><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E352-MJMAIN-28" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E352-MJMATHI-69" x="389" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E352-MJMAIN-29" x="734" y="0"></use></g><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E352-MJMATHI-6A" x="809" y="-430"></use></g></g></svg></span><script type="math/tex" id="MathJax-Element-300">\frac{\partial }{\partial {\theta_{j}}}\left( \theta^Tx^{(i)}  \right)=x^{(i)}_j</script></p><p>所以有：</p><p><span class="MathJax_Preview"></span><span class="MathJax_SVG" id="MathJax-Element-237-Frame" tabindex="-1" style="font-size: 100%; display: inline-block;"><svg xmlns:xlink="http://www.w3.org/1999/xlink" width="65.095ex" height="4.445ex" viewBox="0 -1208.2 28026.8 1913.9" role="img" 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x="18494" y="0"></use><g transform="translate(19495,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E640-MJMATHI-79" x="0" y="0"></use><g transform="translate(499,362)"><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E640-MJMAIN-28" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E640-MJMATHI-69" x="389" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E640-MJMAIN-29" x="734" y="0"></use></g></g><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E640-MJMAIN-29" x="20890" y="0"></use><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E640-MJMAIN-22C5" x="21501" y="0"></use><g transform="translate(22002,0)"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E640-MJMATHI-78" x="0" y="0"></use><g transform="translate(572,521)"><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E640-MJMAIN-28" x="0" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E640-MJMATHI-69" x="389" y="0"></use><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E640-MJMAIN-29" x="734" y="0"></use></g><use transform="scale(0.707)" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#E640-MJMATHI-6A" x="809" y="-430"></use></g></g></svg></span><script type="math/tex" id="MathJax-Element-474">\frac{\partial }{\partial {\theta_{j}}}J(\theta) = -\frac{1}{m}\sum\limits_{i=1}^{m}{\frac{\partial }{\partial {\theta_{j}}}f(\theta)}=\frac{1}{m} \sum\limits_{i=1}^{m} (h_\theta(x^{(i)}) - y^{(i)}) \cdot x_j^{(i)} </script></p><p>&nbsp;</p><h2><a name='header-n198' class='md-header-anchor '></a>6.6 进阶优化(Advanced Optimization)</h2><p>&nbsp;</p><h2><a name='header-n201' class='md-header-anchor '></a>6.7 多类别分类: 一对多(Multiclass Classification: One-vs-all)</h2><h1><a name='header-n202' class='md-header-anchor '></a>7 Regularization</h1><h2><a name='header-n203' class='md-header-anchor '></a>7.1 The Problem of Overfitting</h2><h2><a name='header-n204' class='md-header-anchor '></a>7.2 Cost Function</h2><h2><a name='header-n205' class='md-header-anchor '></a>7.3 Regularized Linear Regression</h2><h2><a name='header-n206' class='md-header-anchor '></a>7.4 Regularized Logistic Regression</h2></div>
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diff --git a/week3.md b/week3.md
index 797a4c9..c4621d3 100644
--- a/week3.md
+++ b/week3.md
@@ -10,11 +10,11 @@
 - 金融欺诈判断
 - 肿瘤诊断
 
-肿瘤诊断问题：
+讨论肿瘤诊断问题：
 
 ![](image/20180109_144040.png)
 
-肿瘤诊断问题是一个**二元分类问题(binary class problems)**，则定义 $ y \in\lbrace 0, 1\rbrace$，其中 0 表示**负向类(negative class)**，代表恶性肿瘤("-")，1 为**正向类(positive class)**，代表良性肿瘤("+")。如图，定义最右边的样本为**偏差项**。
+肿瘤诊断问题的目的是告诉病人**是否**为恶性肿瘤，是一个**二元分类问题(binary class problems)**，则定义 $ y \in\lbrace 0, 1\rbrace$，其中 0 表示**负向类(negative class)**，代表恶性肿瘤("-")，1 为**正向类(positive class)**，代表良性肿瘤("+")。如图，定义最右边的样本为**偏差项**。
 
 在未加入偏差项时，线性回归算法给出了品红色的拟合直线，若规定
 
@@ -26,21 +26,21 @@ $h_\theta(x) \lt 0.5$ ，预测为 $y = 0$，即负向类。
 
 
 
-接下来加入偏差项，线性回归算法给出了靛青色的拟合直线，如果阈值仍然为 0.5，可以看到算法在某些情况下会给出完全错误的结果。
+接下来加入偏差项，线性回归算法给出了靛青色的拟合直线，如果阈值仍然为 0.5，可以看到算法在某些情况下会给出完全错误的结果，对于癌症、肿瘤诊断这类要求预测极其精确的问题，这种情况是无法容忍的。
 
-不仅如此，线性回归算法的值域为 $R$，则当线性回归函数给出诸如 $h = 10000, h = -10000$ 等很大/很小(负数)的数值时，结果 $y \in \lbrace 0, 1\rbrace$，这显得非常怪异。
+不仅如此，线性回归算法的值域为 $R$，则当线性回归函数给出诸如 $h_\theta(x) = 10000, h_\theta(x) = -10000$ 等很大/很小(负数)的数值时，结果 $y \in \lbrace 0, 1\rbrace$，这显得非常怪异。
 
 
 
-区别于线性回归算法，逻辑回归算法是一个分类算法，**其输出值永远在 0 到 1 之间**，即 $h \in (0,1)$。
+区别于线性回归算法，逻辑回归算法是一个分类算法，**其输出值永远在 0 到 1 之间**，即 $h_\theta(x) \in (0,1)$。
 
 ## 6.2 假设函数表示(Hypothesis Representation)
 
-为了使 $h \in \left(0, 1\right)$，引入逻辑回归模型，定义假设函数
+为了使 $h_\theta(x) \in \left(0, 1\right)$，引入逻辑回归模型，定义假设函数
 $$
-h_\theta \left( x \right)=g(z)=g\left(\theta^{T}X \right)
+h_\theta \left( x \right)=g(z)=g\left(\theta^{T}x \right)
 $$
-对比线性回归函数 $h_\theta \left( x \right)=\theta^{T}X$，$g$ 表示逻辑函数([logistic function][1])，复合起来，则称为逻辑回归函数。
+对比线性回归函数 $h_\theta \left( x \right)=\theta^{T}x$，$g$ 表示逻辑函数([logistic function][1])，复合起来，则称为逻辑回归函数。
 
 逻辑函数是 S 形函数，会将所有实数映射到 $(0, 1)$ 范围。
 
@@ -63,9 +63,9 @@ $\begin{align*}& h_\theta(x) = P(y=1 | x ; \theta) = 1 - P(y=0 | x ; \theta) \ne
 
 ## 6.3 决策边界(Decision Boundary)
 
-决策边界的概念可以帮助我们更好地理解逻辑回归模型。
+决策边界的概念，可帮助我们更好地理解逻辑回归模型的拟合原理。
 
-在逻辑回归中，有假设函数 $h_\theta \left( x \right)=g(z)=g\left(\theta^{T}X \right)$。
+在逻辑回归中，有假设函数 $h_\theta \left( x \right)=g(z)=g\left(\theta^{T}x \right)$。
 
 为了得出分类的结果，这里和前面一样，规定以 $0.5$ 为阈值：
 
@@ -76,11 +76,11 @@ $\begin{align*}& h_\theta(x) \geq 0.5 \rightarrow y = 1 \newline& h_\theta(x) <
 
 ![sigmoid function](image/2413fbec8ff9fa1f19aaf78265b8a33b_Logistic_function.png)
 
-观察可得当 $g(z) \geq 0.5$ 时，有 $z \geq 0$，即 $\theta^TX \geq 0$。
+观察可得当 $g(z) \geq 0.5$ 时，有 $z \geq 0$，即 $\theta^Tx \geq 0$。
 
 同线性回归模型的不同点在于： $\begin{align*}z \to +\infty, e^{-\infty} \to 0 \Rightarrow g(z)=1 \newline z \to -\infty, e^{\infty}\to \infty \Rightarrow g(z)=0 \end{align*}$
 
-直观一点来个例子，${h_\theta}\left( x \right)=g\left( {\theta_0}+{\theta_1}{x_1}+{\theta_{2}}{x_{2}}\right)$ 是下图模型的假设函数。
+直观一点来个例子，${h_\theta}\left( x \right)=g\left( {\theta_0}+{\theta_1}{x_1}+{\theta_{2}}{x_{2}}\right)​$ 是下图模型的假设函数：
 
 ![](image/20180111_000814.png)
 
@@ -90,35 +90,120 @@ $\begin{align*}& h_\theta(x) \geq 0.5 \rightarrow y = 1 \newline& h_\theta(x) <
 
 
 
-上面讨论了逻辑回归模型中线性拟合的例子，下面则是一个多项式拟合的例子，和线性回归中讨论的其实没有多大区别。
+上面讨论了逻辑回归模型中线性拟合的例子，下面则是一个多项式拟合的例子，和线性回归中的情况也是类似的。
 
 为了拟合下图数据，建模多项式假设函数：
 
 ${h_\theta}\left( x \right)=g\left( {\theta_0}+{\theta_1}{x_1}+{\theta_{2}}{x_{2}}+{\theta_{3}}x_{1}^{2}+{\theta_{4}}x_{2}^{2} \right)$
 
-这里取 $\theta = \begin{bmatrix} -1\\0\\0\\1\\1\end{bmatrix}$，决策边界对应了一个在原点处的单位圆，如此便可给出分类结果了，如图中品红色曲线：
+这里取 $\theta = \begin{bmatrix} -1\\0\\0\\1\\1\end{bmatrix}$，决策边界对应了一个在原点处的单位圆（${x_1}^2+{x_2}^2 = 1$），如此便可给出分类结果，如图中品红色曲线：
 
 
 
 ![](image/20180111_000653.png)
 
-当然，通过一些更为复杂的多项式，还能拟合那些图像显得非常怪异的数据，使得决策边界像碗状、爱心状等等。
+当然，通过一些更为复杂的多项式，还能拟合那些图像显得非常怪异的数据，使得决策边界形似碗状、爱心状等等。
 
 
 
 简单来说，决策边界就是**分类的分界线**，分类现在实际就由 $z$ (中的 $\theta$)决定啦。
 
-## 6.4 代价函数(Cost Function)
+## 6.4 损失函数(Cost Function)
 
 上节又留下了个问题，我们怎么知道决策边界是啥样？$\theta$ 多少时能很好的拟合数据？当然，见招拆招，总要来个 $J(\theta)$。
 
+如果直接套用线性回归的损失函数： $J\left( {\theta} \right)=\frac{1}{2m}\sum\limits_{i=1}^{m}{{{\left( h_{\theta} \left({x}^{\left( i \right)} \right)-{y}^{\left( i \right)} \right)}^{2}}}$
 
+其中 $h_\theta(x) = g\left(\theta^{T}x \right)$，可绘制关于 $J(\theta)$ 的图像，如下图
 
-## 6.5 Simplified Cost Function and Gradient Descent
+![](image/20180111_080314.png)
 
-## 6.6 Advanced Optimization
+回忆线性回归中的损失函数，其是一个二次凸函数（碗状），二次凸函数的重要性质是只有一个局部最小点即全局最小点。上图中有许多局部最小点，这样梯度下降算法将无法确定收敛点是全局最优。
 
-## 6.7 Multiclass Classification_ One-vs-all
+![](image/20180111_080514.png)
+
+如果是一个凸函数，可以对其进行最优化分析，这类最优化问题，称为**凸优化问题**。还好，损失函数不止平方损失函数一种。
+
+对于逻辑回归，更换平方损失函数为**对数损失函数：**
+
+$\begin{align*}& J(\theta) = \dfrac{1}{m} \sum_{i=1}^m \mathrm{Cost}(h_\theta(x^{(i)}),y^{(i)}) \newline & \mathrm{Cost}(h_\theta(x),y) = -\log(h_\theta(x)) \; & \text{if y = 1} \newline & \mathrm{Cost}(h_\theta(x),y) = -\log(1-h_\theta(x)) \; & \text{if y = 0}\end{align*}$
+
+则可绘制关于 $J(\theta)$ 的图像如下：
+
+![](image/20180111_080614.png)
+
+如左图，当训练集的结果为 $y=1$（正样本）时，随着假设函数趋向于 $1$，损失函数的值会趋于 $0$，即意味着拟合程度很好。如果假设函数此时趋于 $0$，则会给出一个**很高的损失**，拟合程度**差**，算法会根据其迅速纠正 $\theta$ 值，右图 $y=0$ 同理。
+
+区别于平方损失函数，对数损失函数也是一个凸函数，但没有局部最优值。
+
+## 6.5 简化的成本函数和梯度下降(Simplified Cost Function and Gradient Descent)
+
+由于懒得分类讨论，对于二元分类问题，我们把损失函数**简化**为一个函数： 
+$Cost\left( {h_\theta}\left( x \right),y \right)=-y\times log\left( {h_\theta}\left( x \right) \right)-(1-y)\times log\left( 1-{h_\theta}\left( x \right) \right)$
+
+当 $y = 0$，左边式子整体为 $0$，当 $y = 1$，则 $1-y=0$，右边式子整体为0，也就和上面的分段函数一样了，而一个式子计算起来更方便。
+
+$J(\theta) = - \frac{1}{m} \displaystyle \sum_{i=1}^m [y^{(i)}\log (h_\theta (x^{(i)})) + (1 - y^{(i)})\log (1 - h_\theta(x^{(i)}))]$
+
+向量化实现：
+
+$h = g(X\theta)$，$J(\theta) = \frac{1}{m} \cdot \left(-y^{T}\log(h)-(1-y)^{T}\log(1-h)\right)$
+
+
+
+为了最优化 $\theta$，仍使用梯度下降法，算法同线性回归中一致：
+
+$\begin{align*} & \text{repeat until convergence:} \; \lbrace \newline \; &{{\theta }_{j}}:={{\theta }_{j}}-\alpha \frac{\partial }{\partial {{\theta }_{j}}}J\left( {\theta}  \right) \newline \rbrace \end{align*}$
+
+解出偏导得：
+
+$\begin{align*}& \text{repeat until convergence:} \; \lbrace \newline \; & \theta_j := \theta_j - \alpha \frac{1}{m} \sum\limits_{i=1}^{m} (h_\theta(x^{(i)}) - y^{(i)}) \cdot x_j^{(i)} \; & \text{for j := 0,1...n}\newline \rbrace\end{align*}$
+
+注意，虽然形式上梯度下降算法同线性回归一样，但其中的假设函不同，即$h_\theta(x) = g\left(\theta^{T}x \right)$，不过求导后的结果也相同。
+
+向量化实现：$\theta := \theta - \frac{\alpha}{m} X^{T} (g(X \theta ) - y)$
+
+
+
+**对数损失函数求导的推导过程：**
+
+$J(\theta) = - \frac{1}{m} \displaystyle \sum_{i=1}^m [y^{(i)}\log (h_\theta (x^{(i)})) + (1 - y^{(i)})\log (1 - h_\theta(x^{(i)}))]$
+
+令 $f(\theta) = {{y}^{(i)}}\log \left( {h_\theta}\left( {{x}^{(i)}} \right) \right)+\left( 1-{{y}^{(i)}} \right)\log \left( 1-{h_\theta}\left( {{x}^{(i)}} \right) \right)$
+
+将 $h_\theta(x^{(i)}) = g\left(\theta^{T}x^{(i)} \right)=\frac{1}{1+{{e}^{-{\theta^T}{{x}^{(i)}}}}} $ 带入得
+
+$f(\theta)={{y}^{(i)}}\log \left( \frac{1}{1+{{e}^{-{\theta^T}{{x}^{(i)}}}}} \right)+\left( 1-{{y}^{(i)}} \right)\log \left( 1-\frac{1}{1+{{e}^{-{\theta^T}{{x}^{(i)}}}}} \right)$
+$=-{{y}^{(i)}}\log \left( 1+{{e}^{-{\theta^T}{{x}^{(i)}}}} \right)-\left( 1-{{y}^{(i)}} \right)\log \left( 1+{{e}^{{\theta^T}{{x}^{(i)}}}} \right)$
+
+根据求偏导的性质，没有 $\theta_j$ 的项都消去，则得：
+
+$\frac{\partial }{\partial {\theta_{j}}}\left( \theta^Tx^{(i)}  \right)=x^{(i)}_j$
+
+所以有：
+
+$\frac{\partial }{\partial {\theta_{j}}}f\left( \theta  \right)=\frac{\partial }{\partial {\theta_{j}}}[-{{y}^{(i)}}\log \left( 1+{{e}^{-{\theta^{T}}{{x}^{(i)}}}} \right)-\left( 1-{{y}^{(i)}} \right)\log \left( 1+{{e}^{{\theta^{T}}{{x}^{(i)}}}} \right)]$
+
+$=-{{y}^{(i)}}\frac{-x_{j}^{(i)}{{e}^{-{\theta^{T}}{{x}^{(i)}}}}}{1+{{e}^{-{\theta^{T}}{{x}^{(i)}}}}}-\left( 1-{{y}^{(i)}} \right)\frac{x_j^{(i)}{{e}^{{\theta^T}{{x}^{(i)}}}}}{1+{{e}^{{\theta^T}{{x}^{(i)}}}}}$
+$={{y}^{(i)}}\frac{x_j^{(i)}}{1+{{e}^{{\theta^T}{{x}^{(i)}}}}}-\left( 1-{{y}^{(i)}} \right)\frac{x_j^{(i)}{{e}^{{\theta^T}{{x}^{(i)}}}}}{1+{{e}^{{\theta^T}{{x}^{(i)}}}}}$ 
+$={\frac{{{y}^{(i)}}x_j^{(i)}-x_j^{(i)}{{e}^{{\theta^T}{{x}^{(i)}}}}+{{y}^{(i)}}x_j^{(i)}{{e}^{{\theta^T}{{x}^{(i)}}}}}{1+{{e}^{{\theta^T}{{x}^{(i)}}}}}}$
+$={\frac{{{y}^{(i)}}\left( 1\text{+}{{e}^{{\theta^T}{{x}^{(i)}}}} \right)-{{e}^{{\theta^T}{{x}^{(i)}}}}}{1+{{e}^{{\theta^T}{{x}^{(i)}}}}}x_j^{(i)}}$
+$={({{y}^{(i)}}-\frac{{{e}^{{\theta^T}{{x}^{(i)}}}}}{1+{{e}^{{\theta^T}{{x}^{(i)}}}}})x_j^{(i)}}$
+$={({{y}^{(i)}}-\frac{1}{1+{{e}^{-{\theta^T}{{x}^{(i)}}}}})x_j^{(i)}}$
+$={\left({{y}^{(i)}}-{h_\theta}\left( {{x}^{(i)}} \right)\right)x_j^{(i)}}$
+$={\left({h_\theta}\left( {{x}^{(i)}} \right)-{{y}^{(i)}}\right)x_j^{(i)}}$
+
+则可得对数损失函数的导数：
+
+$\frac{\partial }{\partial {\theta_{j}}}J(\theta) = -\frac{1}{m}\sum\limits_{i=1}^{m}{\frac{\partial }{\partial {\theta_{j}}}f(\theta)}=\frac{1}{m} \sum\limits_{i=1}^{m} (h_\theta(x^{(i)}) - y^{(i)}) \cdot x_j^{(i)} $
+
+
+
+## 6.6 进阶优化(Advanced Optimization)
+
+
+
+## 6.7 多类别分类: 一对多(Multiclass Classification: One-vs-all)
 
 # 7 Regularization
 ## 7.1 The Problem of Overfitting